Antibody Building: Does Tapping the Body's Other Immune System Hold the Key to Fending Off HIV Infection?

Antibody Building: Does Tapping the Body's Other Immune System Hold the Key to Fending Off HIV Infection?

 

NIH researchers may have found antibodies that can neutralize most varieties of HIV, blocking it before it infects healthy cells. But stimulating their sparse natural production remains a hurdle in developing an antiviral therapy

By Bob Roehr

INNATE ANTIBODIES: Atomic structure of the antibody VRC01 (blue and green) binding to HIV (grey and red). The precise site of VRC01-HIV binding (red) is a subset of the area of viral attachment to the primary immune cells HIV infects.
NIAID VRC

Scientists at the National Institutes of Health have identified long-sought and elusive broadly neutralizing antibodies to HIV in a pair of papers published in the July 9 issue of Science. These proteins produced by the innate immune system are crucial for creating a preventive vaccine, and could also have therapeutic uses developed in the coming years or decades.
Variations in individuals' innate and adaptive immune systems can dramatically affect responses to infection—HIV is no exception. The result generally can be shown as a bell curve, with a group of people whose disease progresses rapidly, a broad middle segment who progress typically, and a small group of "elite controllers" whose immune systems are quite effective at containing HIV viral replication.
The quest to figure out why has focused primarily on the adaptive immune system, because CD4+ and CD8+ T cells have a clearly demonstrated capacity to kill cells infected with HIV. But that response only arises some days, weeks and even months after a person has been exposed to HIV and the virus has integrated itself into cellular DNA, establishing lifelong infection. The adaptive immune response can only contain an established infection, it cannot prevent that infection from occurring at its onset.
The innate immune system is the first line of defense against infection. It attacks at the initial exposure to a pathogen, and can prevent the establishment of infection—and HIV is no exception. But there are a number of reasons why it has proved difficult to identify components of the innate immune response that can neutralize the deadly virus.
HIV transmission is not very efficient. Exposed persons may avoid infection for a variety of mechanical (barrier) and biological reasons, such as the virus's failure to penetrate to the surface of mucosal tissue or dendritic cell difficulties in latching onto the virus to carry it to a lymph node. So it is challenging to conclusively identify the contribution of a specific innate immune response that can prevent an initial infection.
Over the years, it has become clear that there are factors other than CD4+ and CD8+ T cells that help to control the virus in at least a portion of those infected with HIV.
Researchers have identified several antibodies that can neutralize the virus. Most of them bind weakly to small, often deep, pockets on the virus. In most instances, once infection becomes established rapidly mutating HIV evolves resistance to those narrowly focused antibodies, often by adding glycans or sugars to its outer envelope, which shields or blocks antibody access to the binding site.
What is needed is an antibody that binds strongly to a surface site on the virus, and which cannot be easily blocked. It is also important that the binding site is greatly conserved across the many strains of HIV.
Researchers at NIH Vaccine Research Center (VRC) decided to look at neutralizing antibodies in the blood of persons who are able to better control HIV infection. Elite controllers were not part of the mix because they seem to control HIV through their adaptive immunological system T cell mechanisms.
Using sophisticated reverse-engineering techniques, the researchers identified three proteins that are broadly neutralizing, which they labeled VRC01, VRC02 and VRC03. They also isolated the B cells that produced them.
The first two antibodies have very similar chemical structures and bind to HIV's gp120 trimer spike on its surface. The virus uses the trimer to link up with a CD4 receptor, which is the first of many steps taken to enter and infect a host cell. The antibody and gp120 spike bind in a way that is, in part, similar to the way that the spike and CD4 receptor bind.
As a result, VRC01 and VRC02 binding is particularly long and strong compared with the bonds formed by other antibodies. Further, the binding site on the gp120 spike is well exposed and not likely to become blocked by the addition of sugars to the viral envelope.
The two antibodies neutralized 91 percent of the 190 different HIV isolates that the team tested. Those isolates represent all of the various clades or strains of HIV present worldwide, says John Mascola, one of the VRC research team leaders. Also, the antibodies were able to neutralize all of the limited number of HIV variants that are transmitted sexually—a key point, because 80 percent of all new infections result from sexual activity.
VRC01 and VRC02 occur naturally and are produced by what are called RSC3 memory-specific B cells, an extremely rare component of the innate immune system. Using flow cytometry, the NIH team could isolate only 29 of those cells from among the 25 million cells that they screened. Furthermore, the proteins produced by those B cells often are immature and it appears that the proteins must undergo a series of combinations before they become functional VRC01 or VRC02.
X-ray crystallography allowed the researchers "to identify the [antibody's] binding site down to the atomic level structure…. It is a particularly invariant part of the CD4 binding site, which is exposed," Mascola says. He calls that knowledge "a blueprint from which to design new vaccines. It allows us to try to design a protein that mimics and presents that specific site to the immune system" to stimulate B cells "to crank out the antibody."
Mascola acknowledges the complex nature of the VRC01 and VRC02 antibodies and their low naturally occurring numbers may prove to be an obstacle to developing a vaccine. It is too early to understand all of the issues surrounding the stimulation of antibody production and the concentration necessary to afford protection from infection.
The VRC research team has designed vaccine antigens that already are in preclinical study in small animals. If those prove successful, the work may advance into a monkey model, although it is not completely clear how monkeys can control the simian version of HIV and not progress to advanced disease. The identification of VRC01 and VRC02 may also help to advance a better understanding of the disease in monkeys.
Mascola says these discoveries also may lead to development of a "therapeutic vaccine" or immune-based therapy that helps train the innate immune system of an HIV-infected person to better control the virus without the use of drugs.
It may be possible to mass-produce these antibodies for passive administration as an adjunct or substitute for current small molecule drugs used to treat HIV. And if production costs can be reduced sufficiently there may be a role for them in topical microbicides as a preventative for HIV exposure.

HIV Antibody The Test

HIV Antibody The Test

1. How is it used?
2. When is it ordered?
3. What does the test result mean?
4. Is there anything else I should know?

How is it used?

HIV antibody testing is used to screen for and diagnose HIV infections. Early treatment of HIV infection and immune system monitoring can greatly improve long-term health. Also, knowing your HIV status may help you change behaviors that would put you and others at risk.

Antibodies to the HIV virus are often detected by a screening test called an ELISA. The ELISA test is repeated if positive. The ELISA method is very sensitive but requires another test, a Western Blot, to confirm the results because false positives can occur. These tests can be done on blood, urine or oral sample in a doctor’s office or a local clinic. There are several rapid tests available in which results are generated in about 20 minutes. However, these too must have confirmatory testing before a final diagnosis can be made.

There is also a home collection kit approved by the US Food and Drug Administration (FDA) that is available for HIV antibody testing. This allows you to take a sample of your blood from a finger at home and mail it to a testing center. You would then hear your results later over the phone, along with appropriate counseling. There are, however, no available tests that can be performed at home. (For more, see the article on Home Testing and the FDA webpage on HIV home tests.)

 

When is it ordered?

Several organizations, including the Centers for Disease Control (CDC), American College of Physicians (ACP), and HIV Medicine Association (HMA) recommend that anyone over the age of 13 be screened for HIV.

Antibody testing for HIV is especially important if you are in a high risk group or if you think you may have been exposed to HIV. Testing is recommended if:

• You are sexually active (three or more sexual partners in the last 12 months).
• You received a blood transfusion prior to 1985, or a sexual partner received a transfusion and later tested positive for HIV. For more information, see the CDC webpage How safe is the blood supply in the US?
• You are uncertain about your sexual partner’s risk behaviors.
• You are a male who has had sex with another male.
• You have used street drugs by injection, especially when sharing needles and/or other equipment.
• You have a sexually transmitted disease (STD).
• You are a health care worker with direct exposure to blood on the job.
• You are pregnant. (There are now treatments that can greatly reduce the risk that a pregnant woman who has HIV will give the virus to her baby.)
• You are a woman who wants to make sure you are not infected with HIV before getting pregnant (see Pregnancy: Pre-conception, HIV).

 

What does the test result mean?

A healthy individual has no antibodies to HIV. However, a negative screening test means only that there is no evidence of disease at the time of the test. It is important for those who are at increased risk of HIV infection to have screening tests performed on a regular basis to check for possible exposure to the virus.

If you test positive for HIV antibodies on both the ELISA and the Western Blot tests, you are considered to be infected with HIV. HIV cannot be cured, but early diagnosis allows for treatment that can help to suppress levels of virus in your body (viral load) and slow progression of the disease.

 

Is there anything else I should know?

Antibody testing will not detect HIV immediately after exposure, during the window before the development of antibodies. If you are tested too soon, your result may be negative despite the fact that you are infected (false negative). In this case, you may be tested using a p24 antigen test that can detect actual viral protein in the blood 1 to 3-4 weeks after exposure or an HIV RNA test (viral load) that detects the presence of the virus, not the antibody response to it. Or you should have another HIV antibody test in 3–6 months from the time of a possible exposure to the virus.

HIV Antibody Common Questions

HIV Antibody Common Questions

 

Common Questions

1. What are the symptoms of HIV infection?
2. When does AIDS develop?
3. What are the treatments for HIV/AIDS?
4. Should I tell anyone else of my test results?
5. How confidential are HIV test results?
6. Can you use the HIV antibody test to detect HIV in newborns?
7. Are there HIV testing methods other than a blood test?

1.  What are the symptoms of HIV infection?

The only reliable way to tell if you are infected with HIV is to get tested. This is because many people with HIV do not experience symptoms for years after the initial infection or have symptoms that are very similar to symptoms of other illnesses. Click here for information from the CDC on symptoms of HIV infection.

 

2.  When does AIDS develop?

Symptoms of the initial HIV infection can mimic those of influenza and other viral infections. The term AIDS applies to the most advanced stages of HIV infection. According to the CDC, AIDS is diagnosed when your CD4 T-cell count drops below 200 cells per cubic millimeter of blood or when you have HIV and an AIDS-related illness such as tuberculosis. Click here for more information from the CDC.

 

3.  What are the treatments for HIV/AIDS?

Currently, there is no cure for HIV or AIDS. However, there are therapies that can help. The CDC’s booklet, Living with HIV/AIDS, is available online. The FDA also offers an online list of FDA-approved therapies. Early treatment is recommended.

 

4.  Should I tell anyone else of my test results?

Yes. If you test positive for HIV, it is important that you tell your health care providers as well as all current and future sex partners and/or anyone with whom you share needles. Counseling services are often available from the clinic that performed the test or from your health care provider that will help you to inform the people who need to know.

 

5.  How confidential are HIV test results?

Your HIV status, like other medical conditions and test results, is protected by the HIPAA Privacy Rule and cannot be shared with friends, family, or employers without your written permission. Your HIV status may be shared with your health care providers who have a “need to know” in order to treat you. Also, in order to determine the incidence of HIV and to provide appropriate prevention and care services, all new cases of HIV are reported to state and local health departments.

Certain testing centers provide either anonymous (your name is never given) or confidential (your name is given but kept private) HIV testing and counseling. The FDA has approved one home testing device that allows you to remain anonymous and to get confidential results. You can also contact your state, county, or city health department to find out where testing may be available.

 

6.  Can you use the HIV antibody test to detect HIV in newborns?

No. Because maternal antibodies are transferred from mother to baby and stay in the newborn’s system for 6–12 months, a different test must be used. This test is called the HIV DNA test

 

7.  Are there HIV testing methods other than a blood test?

Yes. Methods that test either oral fluid or urine are available in some locations. The CDC has more information on the different types of HIV screening tests available in the U.S. (click here).

HIV Antibody At a Glance

HIV Antibody At a Glance

---

Also known as: AIDS test; AIDS screen; HIV serology

Formal name: Human immunodeficiency virus antibody

 

 

Why Get Tested?

To determine if you are infected with Human immunodeficiency virus (HIV)

When to Get Tested?

One month to three months after you think you may have been exposed to the virus; the average time for the antibody to be detected is two weeks after exposure to the virus

Sample Required?

A blood sample collected from a vein in your arm; there are also tests available that can be performed on urine and/or oral fluid

Test Preparation Needed?

None

DO HIV ANTIBODY TESTS PROVE HIV INFECTION?

DO HIV ANTIBODY TESTS PROVE HIV INFECTION?

By Valendar Turner

Department of Emergency Medicine, Royal Perth
Hospital, Perth, Western Australia

What evidence authenticates a positive HIV antibody test as proof of HIV infection? This question has greatly interested me because those of us who work in Emergency Medicine spend a considerable part of our lives exposed to other people's blood and body fluids, a circumstance which, according to the experts, places us under constant threat of death from AIDS. Ironically, if the experts are right, the life we save may cost us our own and it's little wonder that some of us have pursued the question of proving HIV infection to the very limits. From the early days of AIDS I was fortunate to collaborate with Eleni Eleopulos, a Biophysicist at the Royal Perth Hospital, John Papadimitriou, Professor of Pathology at the University of Western Australia, and other colleagues, and in one of our papers, published in June 1993 in the journal Bio/Technology [1], we were compelled to confront many unsettling conclusions about the HIV antibody tests, none of which accord with current wisdom. Some of these I would like to share with you today.

The HIV antibody tests do not detect a virus. They test for any antibodies that react with an assortment of proteins experts assure us are unique to HIV which, almost everyone agrees, is a retrovirus and the cause of AIDS [2]. What happens is this: A sample of blood serum is incubated with a mixture of these proteins in a test called an ELISA, an acronym for Enzyme Linked Immunosorbent Assay. The ELISA is positive if the solution changes colour thereby indicating a reaction between the proteins in the test kit and the patient's antibodies. However, according to many experts, the ELISA is not specific meaning it may react in the absence of HIV infection. In response to this, testing authorities have developed strategies such as repeat testing of all positive ELISAs and following up those twice positive with a third but different antibody test known as the Western blot. In the Western blot the "HIV" proteins, about ten of them, are located at discrete spots in a paper strip, rather like the one your doctor uses to perform multiple tests on your urine. Serum is added and wherever there is a reaction a colour change occurs which shows up as a dark band. The test is read by noting which bands show up, in other words, which proteins react. Certain combinations of bands are defined as a positive test. It is enigmatic that the location and number of bands required for a positive Western blot varies around the world. They may even vary between laboratories within the same city. In Australia four bands are required, in Canada and much of the United States, three bands suffice. And in Africa two will do. In the US Multicenter AIDS Cohort prospective study involving several thousand gay men, one "strong" band was deemed sufficient. If each of the above indicates HIV infection then HIV must cause different populations of antibodies to appear in different places. I don't know about you but to me that sounds very odd. But at least it gives some Africans a way out. All an African has to do is have a test in Australia because two bands would not be considered positive here. Nevertheless, in spite of lack of standardisation and other problems such as reproducibility, the Western blot is accepted to be in excess of 99.9% specific and if positive is regarded synonymous with HIV infection. In some countries similar claims are now made for the HIV ELISA without recourse to the Western blot.

The rationale for the use of antibody tests is as follows: The immune system has the ability to detect foreign agents and to respond by producing antibodies which react with those agents. However, this does not work in reverse. By that I mean the observation of an antibody reaction with a particular agent is not automatic proof that the antibody was produced in response to that agent. The problem is that antibodies indulge in casual and indiscriminate relationships. They are in fact promiscuous. Antibodies meant for one agent may react with another agent, a perfect stranger. Or, if you want it put technically, there is ample evidence, some of the best in fact comes from the Pasteur Insititute, that antibody molecules, even the most pure, the monoclonal antibodies, are not monospecific and cross-react with other, non-immunising antigens. Here are some examples to illustrate this most crucial fact. Firstly, in a study of 1.2 million applicants for US military service [3], of the 1% or 12,000 who had first time positive HIV ELISAs, only 2000 were ultimately shown to be also WB positive and thus, according to the authors, HIV infected. That left 10,000 positive ELISAs which must have reacted for reasons other than "HIV antibodies", a fitting testimonial to the problem caused by cross-reacting antibodies. Secondly, there is the tantalising data reported in 1990 about dogs. Writing in the journal Cancer Research, Strandstrom and colleagues reported that 72/144 (50%) of dog blood samples "obtained from the Veterinary Medical Teaching Hospital, University of California, Davis" tested in commercial Western blot assays, "reacted with one or more HIV recombinant proteins [gp120--21.5%, gp41--23%, p31--22%, p24-- 43%]" [4]. Assuming Californian dogs are not infected with HIV (as did the authors) one must conclude these data are further proof of antibody cross reactivity to many of the "HIV" proteins. What all this means is that you're not necessarily infected with what your antibodies appear to tell you. This can be brought home by two further examples. Firstly, some AIDS patients have antibody reactions with laboratory chemicals but no one claims AIDS patients are infected with laboratory chemicals. Secondly, as an example removed from AIDS, the antibody test for glandular fever relies on the fact that patients with glandular fever make antibodies that react with the red blood cells of sheep and horses. But these patients are not infected with animal blood and animal blood does not cause glandular fever. Bearing all these examples in mind it is painfully obvious we cannot pronounce someone infected with what is regarded as a lethal human retrovirus merely because we observe an antibody reaction. Before we pronounce any such reactions indicative of HIV infection and long before we introduce the test into routine clinical practice, we must exact solid proof of precisely why these reactions take place. In doing we must not forget that biology is not mathematics and despite our clever technology, in biology still we must stoop to the relative ignominy of empirical proofs. Or, as Plato said, "experiential data must always be interpreted in the light of what Nature has revealed".

In Science we must constantly resist the temptation to stray beyond our data and in that spirit I put it to you there are only two pieces of information which can be gleaned from an antibody test (for mathematical purists it's only one piece of information). Either you see a reaction or you don't. That's all. You don't see antibodies with labels attached saying what produced them. You cannot construe the genesis of antibodies by observing changing colours in a test-tube. The cardinal problem scientists face when ascribing meaning to a set of antibody reactions is how can they tell whether the reaction is caused by a real antibody or a ring in? One whose proper partner is something else but caught in a compromising act? In this context it is proper for a disinterested scientist to allow for the possibility that there are no real HIV antibodies whatsoever, that they're all pretenders. When the only information is a reaction, and that reaction has more than one possible cause, as is the case with an antibody test, you need extra information before you can ascribe a particular outcome. So, if you want to claim an antibody reaction signals a particular outcome, such as HIV infection, first you have to prove it. And just before we get to crunch time consider this little morsel. AIDS patients are exposed to many foreign agents are known to have antibodies reacting with dozens of different substances and it makes perfect sense that the more antibodies there are the more chance there will be some that will spoil the test. What this means is that in the very patients you suspect of harbouring a virus there exists the precise circumstances, lots of potentially cross reacting antibodies, which make it imperative to sort out what is really going on.

What's the solution or, more to the point, what's the problem? The problem is how do you know, when you witness an antibody reaction, that is, a positive test, HIV is present too? After all, that's what you really want the test to tell you and the question on the patient's lips is bound to be "Is HIV infection the only cause of a positive test? If's there's something else I'd rather have that, thank you very much". In technical terms the patient's hopes are hanging on the specificity of the test. Let me first explain what is meant by 100% specificity. One hundred per cent specificity means that positive tests only occur in HIV infected people. That's the same as saying positive tests never occur in uninfected people. And that's the same as saying all uninfected people have a negative test. This leads us to the formal, mathematical definition of specificity which is the number of negative tests in a large group of individuals who do not have HIV infection. If 100% of one thousand people who do not have HIV infection are seronegative the specificity is 100%. If one uninfected person is seropositive the specificity is reduced to 999/1000 or 99.9% by virtue of a lone false positive. Thus, to determine the specificity of an antibody test we need two pieces of data. The numbers of persons with negative tests and the numbers of persons with no HIV infection. By the way, and I'm sure it's obvious, the false-positive rate is (1-the specificity). An experiment to find the specificity also gives the false positive rate and vice versa. How should we design an experiment to discover this important data?

Firstly, since the underlying problem is largely one of deciding between bona fide and cross-reacting antibodies we must include in our sample persons who are likely to have a large repertoire of antibodies to agents other than HIV. The more the merrier. Thus we must include persons who are sick and who have diseases similar to AIDS but not AIDS. Secondly, we need a way of determining the presence or absence of HIV infection. Obviously, this can't be the antibody test itself because that's what we're trying to validate. When we measure specificity we are trying to find out how often reactions occur in individuals who do NOT have HIV infection. Rather surprisingly, in the AIDS literature, the specificity of the HIV antibody tests has been evaluated by testing for reactions in healthy individuals such as blood donors. These persons are chosen as de factos for the absence of HIV infection. Under these circumstances few if any positive reactions are found but this is not necessarily, as the HIV/AIDS experts claim, because the tests are highly specific. In fact, this is the wrong experiment and wrong for two reasons. Firstly, healthy people do not have large number or variety of antibodies to react in the first place. That goes with being healthy. That's why we put them in the Army and let them donate blood. There are simply not enough antibodies available to measure the propensity for unwanted reactions. It's like going to a party where hardly anyone is hogging the Guinness because there's hardly any people. Secondly, good health cannot be used as a de facto for the absence of HIV infection any more than good health can be used as a de facto for the absence of gall stones, kidney stones, pregnancy, hydatid cysts, deep vein thrombosis, cerebral aneurysms, pathogenic bacteria or coronary artery disease. The practice, widely adopted by HIV/AIDS experts, of appraising HIV antibody tests by testing thousands of healthy blood donors, also creates an enormous dilemma. If healthy people are regarded as a de facto gold standard for the absence of HIV infection, counting the occasional one or two who do react as false-positives, by what criteria can similar or even the same individuals be regarded as infected at some future date? One week the same individual may be tested as member of a cohort of healthy blood donors and the following week when he or she requests an examination for Life Insurance or attends a doctor for a checkup. Is this person HIV infected or not? Does the outcome depend solely upon who you are and which door you knock on?

Back to the problem of validation. We select our thousand people who are sick and let's make sure we include some who have diseases similar to AIDS and let's include a few healthy persons and some cases of AIDS as well. You never know, we might be in for a big surprise. We might find some AIDS patients too are antibody positive in the absence of HIV infection. In fact, if you read Gallo's May 1984 Science papers, where it is claimed HIV was proven to be the cause of AIDS, HIV could be "isolated" in less than half the AIDS cases. Let us return to our experiment. Most of the people selected will have lots of antibodies and this will give the test a fair run for its money. There'll be a lot more people at this party. But hold on, if HIV causes AIDS, and some of our patients have AIDS-like diseases or even if they don't, even those who are healthy, how do we get past the sticky problem of knowing who is or is not infected with HIV? We don't want to include them in our analysis because we want to evaluate the test when there is no HIV infection. I know by now many of you will have the correct answer. It's obvious isn't it? You have to use HIV itself. You simply divide your blood sample in two. One to test for the antibody reactions and the other to try and isolate HIV. If you want to know what the HIV antibody tests tell you about HIV infection you compare the reactions with what you are trying to measure. Not with pumpkins. The only way to distinguish between real reactions and cross-reactions is to use HIV isolation as an independent yardstick or gold standard.

What are the results of such an experiment? How many of an appropriately chosen, thousand patients from whom HIV cannot be isolated at the same time have an antibody reaction? I can't tell you that because, bizarre as it may sound, twelve years since the discovery of HIV and ten years since the development of the HIV antibody tests, this experiment has not been done. We don't know how many positive tests occur in the absence of HIV infection, it might be none or it might be all of them. Nobody knows. There is no proof of the specificity of the HIV antibody tests for HIV infection.

What if someone decided to do this experiment? Is it feasible? That's hard to say because it depends on how much importance you place on the precision of defining HIV infection. Ultimately this can only be defined by the isolation a unique retrovirus. The word isolation comes from the Latin word "insulatus" meaning "made into an island". It refers to the act of separating an object from everything else that is not that object. Like solitary confinement. The rules of retrovirus isolation are now old. All the HIV experts should know them. They were developed in the several decades preceding the beginning of the AIDS era in 1981 and were thoroughly discussed at a meeting held at the Pasteur Institute in 1973 and attended by now leading HIV/AIDS researchers including Barre-Sinoussi and Chermann. These are a set of rules which credibly achieve the aim of separateness. The problem is that no claim of HIV isolation yet presented fulfils either the island concept or follows these rules. None of these claims even fulfils the initial and most basic of these rules, the requirement to obtain and electron micrograph of the material which is present at a sucrose density gradient of 1.16 gm/ml. In fact no claim of HIV isolation is isolation. All such claims are based on a set of phenomena ("HIV" proteins such as p24, reverse transcriptase enzyme activity, "HIV" particles, "HIV PCR") detected in cultures of tissues of AIDS patients none of which is even specific for retroviruses. And without isolation who can say whether the proteins used in the HIV antibody tests are unique to HIV? These facts are accepted by Philip Mortimer and his colleagues from the UK Public Health Laboratory Service: "Experience has shown that neither HIV culture nor tests for p24 antigen are of much value in diagnostic testing. They may be insensitive and/or non-specific"[5].

Yes, I know that we have all been shown pictures of something called HIV but that should come as no surprise because, in the extensive retrovirology literature, retrovirus-like particles are commonplace. For a start try insects, reptiles, fish and tapeworms. They are also found in the majority of healthy human placentas and while it is true that electron microscopy reveals retroviral-like particles in 90% of enlarged lymph nodes from AIDS patients, the identical particles can also be found in 90% of enlarged lymph nodes from patients who do not have AIDS and who are not at risk for developing AIDS [6]. If the particles seen in lymph nodes from AIDS patients are HIV as the AIDS experts assure us, what are the particles seen in the lymph nodes of patients who are not at risk from AIDS and what is their relationship to the plethora of other particles seen in cultures of tissues from AIDS patients?

Wait on, I hear some ask, what about the polymerase chain reaction or PCR? For those who don't know, this is a new and very sensitive technique for finding genetic blueprints. Surely this can put us straight about the antibody tests? Not so I'm afraid. To perform the PCR you need to begin with a piece of RNA or DNA which you can say for certain belongs to the HIV genome. To obtain the HIV genome first you need to isolate an HIV particle. That's where the HIV genome comes from and that is the only way to know the RNA or DNA actually belongs to the virus. Even the most charitable interpretation of the data available to date does not show that a unique retrovirus, HIV, has been isolated. Furthermore, even if one assumes that the process of selecting the RNA and DNA molecules (molecular probes) used in the PCR are from the HIV genome there are still many problems with the use of the PCR to prove HIV infection. For a start, at best, the PCR detects single genes and most often, only bits of genes. If your PCR finds two or three genetic fragments out of a possible dozen complete genes is this proof that you have all the genes? The whole genome? No it is not, and in fact HIV experts admit that the majority of HIV genomes studied are defective. This means they are incomplete and could never orchestrate the synthesis of a viral particle. Even if all genomes were complete let's not imagine for a moment having the plans means you've built the house. Basic retrovirology long teaches us you can carry a whole retroviral genome around inside your cells all your life without ever making a viral particle. And in 1992, in the only study of its type, French researchers found the HIV PCR non-reproducible and the agreement between the PCR and the HIV Western blot was found to vary between 40-100% and was especially poor when fragments of more than one gene were sought [7]. In this study there were several PCR negative/HIV positive as well as several PCR positive/HIV negative samples. In other words, the two tests don't fit. As far as which test proves HIV infection, you pay your money and you take your pick.

Finally, a specificity in excess of 99.9% sounds pretty damming, but is it? What if you were found to have a positive HIV antibody test? What is your chance of being truly HIV infected, and not a false-positive? To answer this let's imagine a population of one million people where somehow, by authentic isolation studies if you like, we know 1/1000 persons are HIV infected. That's the prevalence touted for Australia. Let's also assume that there is definite proof, measured against a viral isolation gold standard, that the HIV antibody tests are 99.9% specific for HIV infection. If the test is also 100% sensitive it will detect all of the 1000 infected people. However, 0.1% (1-specificity) of the 999,000 non-infected remainder will also be seropositive. That's another 999 people making a total of 1999 positive tests, 1000 who are infected and 999 who aren't. If you were randomly selected and found to be antibody positive there is only a 50/50 chance you are actually infected. The test will be wrong half the time. But for most of you, we can probably do better than this because most of you are arguably somewhat removed from the risk groups that dominate the statistics. If your prior odds are say, only 1/2000 of being infected, and if we drop the specificity of the test slightly to a mere 99.6%, a positive test will be wrong in 89% of cases, in other words, almost all of the time.

Where does all this leave HIV/AIDS patients? Firstly, the only evidence that HIV is the cause of AIDS is the perception by the AIDS experts of a correlation between antibody reactions and the presence of AIDS-defining diseases. However, for AIDS patients who have had antibody tests and have been diagnosed HIV infected solely on the basis of these tests, we can argue that there is no proof that even one such patient is infected with a virus called HIV. Secondly, in these cases, the tests provide no justification for the administration of potentially toxic drugs like AZT on the basis of a perceived anti-viral activity. Certainly the HIV antibody tests confirm that certain diseases are AIDS rather than just those diseases but this can construed as an artefact of definition. The only scientific conclusion we are permitted to make is that in some but not all well defined at risk individuals, there is a correlation between antibody reactions, whatever their raison d'etre, and the propensity to develop and die from certain diseases. On the other hand, if you're HIV positive but not in a risk group and especially if you're healthy, any pronouncements on your likely outcome will be severely confounded by knowing you are positive, a situation we might describe as twentieth century bone pointing*. And your health may suffer further from the use of medications administered in good faith to kill a virus you may not have. The failure to verify the antibody tests against the gold standard of virus isolation is a serious omission of scientific method. In the absence of such validation these tests should not be used to diagnose HIV infection.

* Bone pointing is a traditional, ritualistic punishment practiced by Australian aborigines. A bone is pointed at an individual as a method of retribution. That individual soon becomes sick and death within weeks or months is an invariable consequence.

This is the text of a radio broadcast for the Australian Broadcasting Commission. Please note: The original text has been amplified.

Addendum

In the entire AIDS literature there is only one study, that of Colonel Donald Burke and his colleagues [3] from the Walter Reed Army Institute, which is widely regarded as the definitive proof of the specificity of the HIV Western blot. Over an eighteen month period Burke and his colleagues tested 1.2 million applicants for US military service. Burke's testing procedure was a progression through two ELISAs and two Western blots. From these data the HIV seroprevalence was found to be 1.48/1000. Burke then retrospectively investigated a highly selected sample of this population in which the seroprevalence was one tenth that of the 1.2 million. This group comprised 135,187 persons aged 17-18 years who resided in rural areas where the cumulative incidence of AIDS was low. Many would assume this group to be no different from healthy blood donors and regard all HIV positives as false positives but Burke and his colleagues' premises were the opposite. Assuming there were true positives amongst healthy, rural American youth and wishing to evaluate the false positive rate and specificity of the Western blot Burke needed to define HIV infection. This was done by performing a panel of four more antibody tests on sera from the 15 out of 135,187 applicants who had already been found twice ELISA and twice Western blot positive. Two of the extra tests were other Western blots and two were similar tests. Any individual positive in all four extra tests, thereby making a total of eight positive antibody tests, was deemed HIV infected. Those who failed any of the extra four tests were deemed non-HIV infected. Of the 15, one failed to complete the panel and thus Burke conceded only one, not fifteen, false-positives. From these data Burke calculated the specificity of the HIV Western blot to be in excess of 99.9%. There are many flaws in this study and they are outlined in reference 1. Here I wish to draw to your attention to the fact that an antibody test, even if repeated and found positive a thousand times, does not prove the presence of a viral infection.

References

1. Eleopulos-Papadopulos E, Turner VF, Papadimitriou JM. 1993. Is a positive Western blot proof of HIV infection? Bio/Technology 11:696-707.

2. Eleopulos-Papadopulos E, Turner VF, Papadimitriou JM. 1993. Has Gallo proven the role of HIV in AIDS? Emergency Medicine [Australia] 5:113-123.

3. Burke DS, Brundage, JF, Redfield, RR et al. 1988. Measurement of the false positive rate in a screening program for human immunodeficiency virus infections. NEJM 319: 961-964.

4. Strandstrom HV, Higgins JR, Mossie K, et al. Studies with canine sera that contain antibodies which recognize human immunodeficiency virus structural proteins. Cancer Res 1990; 50: 5628s-5630s.

5. Mortimer P, Codd A, Connolly J, et al. Towards error free HIV diagnosis: notes on laboratory practice. PHLS Microbiol Digest 1992; 9: 61-64.

6. O'Hara CJ, Groopmen JE, Federman M. 1988. The ultrastructural and immunohistochemical demonstration of viral particles in lymph nodes from human immunodeficiency virus-related lymphadenopathy syndromes. Human Pathology 19:545-549.

7. Defer C, Agut H, Garbarg-Chenon A. 1992. Multicentre quality control of polymerase chain reaction for detection of HIV DNA. AIDS 6:659-663.

Antibody can destroy 90% of HIV strains

Antibody can destroy 90% of HIV strains

Researchers have discovered antibodies that can protect against a wide range of AIDS viruses and said they may be able to use them to design a vaccine against the fatal and incurable virus.

 

Researchers have discovered antibodies that can protect against a wide range of AIDS  viruses  and said they may be able to use them to design a vaccine  against the fatal and incurable virus.

The bodies of some people make these immune system proteins after they are infected with the AIDS virus, when it is too late for them to do much good. But a properly designed vaccine might help the body make them much sooner, the researchers reported in Friday's issue of the journal Science.

"I am more optimistic about an AIDS vaccine at this point in time than I have been probably in the last 10 years," Dr. Gary Nabel of the National Institute of Allergy and Infectious Diseases, who led the study, said in a telephone interview.

Two of the antibodies can attach to and neutralize 90 percent of the various mutations of the human immunodeficiency virus that causes AIDS, Nabel said.

"This is an antibody that evolved after the fact. That is part of the problem we have in dealing with HIV -- once a person becomes infected, the virus always gets ahead of the immune system," Nabel said.

"What we are trying to do with a vaccine is get ahead of the virus."

AIDS infects about 33 million people globally, according to the United Nations AIDS agency UNAIDS. It has killed 25 million people since the pandemic began in the early 1980s and there is no vaccine or cure, although drugs can help control it.

The virus is diifcult to fight in part because it attacks immune system cells and in part because it mutates constantly, making it a moving target for drugs or the immune system.

It has been almost impossible to make a vaccine that will affect the virus. Last September, researchers reported their biggest success yet with a vaccine that appeared to slow the rate of infection by about 30 percent in Thai volunteers but the trial raised many questions.

MOVING TARGETS

Researchers have been looking for parts of the virus that do not mutate so they can design vaccines that will protect against these constantly changing versions.

Nabel's team found two of the antibodies in the blood of a patient infected with HIV who had not become ill despite the infection. Such people are called non-progressors and researchers study their immune systems to find out why they control the virus better than most patients.

They then found the immune system cells called B-cells that made these particular antibodies, using a new molecular device that they invented.

In yet another experiment, they managed to freeze one of the antibodies in the process of attaching to and neutralizing the virus, getting an atomic-level image in a process called x-ray crystallography.

Being able to "see" what the structure looks like could enable researchers to design a vaccine using a process called rational vaccine design, akin to an established technique for making drugs called rational drug design, Nabel said.

It may also be possible to design gene therapy to help patients make these antibodies themselves, or use an older technique that transfuses the antibodies directly.

One of the antibodies, called VRC01, partially mimics the way an immune cell called a CD4 T-cell attaches to a piece of the AIDS virus called gp120, the researchers said.

"The antibodies attach to a virtually unchanging part of the virus, and this explains why they can neutralize such an extraordinary range of HIV strains," Dr. John Mascola, who worked on the study, said in a statement.

"The discovery of these exceptionally broadly neutralizing antibodies to HIV and the structural analysis that explains how they work are exciting advances that will accelerate our efforts to find a preventive HIV vaccine for global use," NIAID director Dr. Anthony Fauci added in a statement.

"In addition, the technique the teams used to find the new antibodies represents a novel strategy that could be applied to vaccine design for many other infectious diseases."

How Accurate is the HIV Antibody Test

How Accurate is the HIV Antibody Test

Q. How accurate are the HIV antibody ELISA and the HIV antibody Western blot?

A. When used together, the results from this two-part testing are greater than 99% accurate. The HIV antibody ELISA is a screening test and the HIV antibody Western blot is a confirmatory test. Results from an HIV antibody ELISA test should never be used alone to report a positive final result.

Q. Do the HIV antibody ELISA and HIV antibody Western blot test for HIV-1 and HIV-2?

A. There are two types of HIV (HIV-1 and HIV-2). Both HIV-1 and HIV-2 have been identified in the United States. The number of known HIV-2 infected persons in the U.S. is less than 100. The estimated number of people in the U.S. infected with HIV-1 is between 650,000 and 900,000.

Some HIV antibody ELISA and HIV antibody Western blot assays detect antibody to both HIV-1 and HIV-2. These are referred to as HIV-1/HIV-2 "combination" tests. Some HIV antibody ELISA and HIV antibody Western blot assays detect antibody primarily to HIV-1 and secondarily to HIV-2. Others detect antibody primarily to HIV-2 and secondarily to HIV-1.

Q. How accurate are HIV antibody tests in detecting the various subtypes of HIV-1?

A. HIV-1 is divided into two groups of subtypes. These two groups are referred to as Group M (major) and Group O (outlier). HIV-1 subtypes of Group M vary, depending on their genetic structure. (3) These include subtypes A through I. In the United States, the predominate HIV-1 subtype is B.

Most antibody tests for detecting HIV-1 were developed with the B subtype of the virus. As the genetic composition of a particular virus diverges from the B subtype, the likelihood that the test will be accurate decreases. Most tests, however, do appear to be able to detect antibody to most strains.

Q. What can cause a false-positive result in an HIV antibody ELISA test?

A. There are many reasons for a false-positive ELISA result. Some of the more common reasons for a false positive are:

• Contamination: In a laboratory, samples may be placed in the wrong testing well; wells containing negative samples may be contaminated from adjacent positive wells; plate washers may malfunction. In addition, treated blood and blood abnormalities have been implicated in false positive reactions.
• False positive reactions have been reported in 19% of people with hemophilia, 13% of alcoholic patients with hepatitis, and 4% of hemodialysis patients.
• Pregnancy. If this is not her first pregnancy, a woman may react positively when she is, in fact, negative.
• History of injection drug use.
• Cross-reactivity with other retroviruses.

Q. What is the expected false-positive rate for an HIV antibody ELISA?

A. The false-positive rate is 1 to 5 per 100,000 assays.

Q. Can a person test HIV-1 antibody negative but be infected with HIV-1?

A. Yes. When people develop antibodies to HIV, they "seroconvert" from antibody-negative to antibody-positive. Depending upon the circumstances of infection, it is estimated that the development of antibodies to

HIV-1 can take between two weeks to six months. During this interval, sometimes referred to as the "window period," a person may test HIV-1 antibody negative and yet be infected with the virus. This is because his/her immune system has not produced enough antibodies for the test to detect.

Q. What does one do if a test result is "indeterminate?" What causes this?

A. The term "indeterminate" relative to HIV testing usually refers to the HIV antibody Western blot assay. The HIV antibody Western blot assay is used on two or more specimens found to be reactive by an HIV antibody ELISA screening assay. (6) Persons who are not at high risk for HIV infection and do not have symptoms, and yet continue to test indeterminate, usually have a very low probability of being infected with HIV. (7) There are many possible reasons for an indeterminate HIV antibody Western blot assay. Some of these reasons might be:

• Prior blood transfusions, even with non-HIV-1 infected blood
• Prior or current infection with syphilis.
• Prior or current infection with malaria parasites.
• Autoimmune disease (e.g. diabetes, Grave's disease, etc.).
• Infection with other human retroviruses (e.g., HIV-2, HTLV I/II).
• Association with "large animals." Animal trainers and veterinarians are sometimes exposed to viruses which do not cause human disease but may interfere with HIV antibody tests.
• Second or subsequent pregnancies in women.

Whether or not persons who test HIV antibody Western blot indeterminate should be retested depends upon their clinical presentation at the time of testing and what risk factors are present for infection. If a person has an indeterminate Western blot assay for HIV-1, several things can be done. These include:

• Run an alternate HIV antibody "confirmatory" assay on the indeterminate HIV antibody Western blot specimen. The FDA has approved an HIV antibody IFA (immunofluorescent assay) procedure as an equivalent confirmatory test to the HIV antibody Western blot.
• Consider running antibody tests for other human retroviruses (HTLV I/II and HIV-2).
• Run tests to identify the presence of the virus. These tests could include HIV DNA PCR, HIV p24 antigen, and HIV culture. (See below)
• Re-test at 3-month intervals for 6 months.

Q. What is meant by a stable indeterminate result?

A. "Stable indeterminate" is a term used to describe a situation in which an individual consistently tests indeterminate 6 months or longer from their last possible exposure. The person should be considered HIV negative unless clinical conditions determine otherwise.

Q. Is the HIV PCR (polymerase chain reaction) test more accurate than the HIV antibody ELISA and HIV antibody Western blot tests? How?

A. Sometimes. The HIV DNA PCR test measures something different than the HIV antibody ELISA and HIV antibody Western blot assays. The HIV DNA PCR test looks for HIV-1 DNA in the white blood cells of a person, whereas the HIV antibody ELISA and HIV antibody Western blot assays measure the immune response to the virus. If a person has a HIV DNA PCR test, the result may be positive even if insufficient antibodies are present for detection by the HIV antibody ELISA.

HIV Antibody Assays Testing

HIV Antibody Assays Testing

HIV Antibody Assays

HIV InSite Knowledge Base Chapter
May 2006

Niel Constantine, PhD, University of Maryland School of Medicine

Introduction

An arsenal of laboratory methods is available to screen blood, diagnose infection, and monitor disease progression in individuals infected by HIV. These tests can be classified into those that: 1) detect antibody, 2) identify antigen, 3) detect or monitor viral nucleic acids, and 4) provide an estimate of T-lymphocyte numbers (cell phenotyping). The focus of this discussion is on antibody detection, the most widely used and, in most situations, most effective way to identify HIV infection.

General Considerations

Tests to detect antibody to HIV can be further classified as: 1) screening assays, which are designed to detect all infected individuals, or 2) confirmatory (supplemental) assays, which are designed to identify individuals who are not infected but who have reactive screening test results. Accordingly, screening tests possess a high degree of sensitivity, whereas confirmatory assays have a high specificity. Tests with high sensitivity produce few false-negative results, whereas tests with high specificity produce few false-positive results. These classes of assays, performed in tandem, produce results that are highly accurate, reliable, and appropriate to protect the blood supply or assist in the diagnosis of HIV infection. Technical errors do occur, however, and there are biologic factors that can limit the accuracy of HIV tests. Therefore, along with the testing process, there is the requirement for an extraordinary and dedicated quality assurance program.(1) Regardless of the results, because laboratory tests are not perfect, they are meant to be a supplement for clinical diagnosis.

Early Detection and the Window Period

Specific antibody to HIV is produced shortly after infection, but the exact time depends on several factors, including host and viral characteristics. Importantly, antibody may be present at low levels during early infection but not at the detection limit of some assays. Using the early-generation tests, antibody could be detected in most individuals by 6 to 12 weeks after infection. Newer-generation assays, including the third-generation antigen sandwich assays, can detect antibody at about 3-4 weeks after infection.(2) This window period before the detection of antibody can be shortened by several days using antigen tests, and by several more days using nucleic acid detection methods.(3) Therefore, in most individuals, the window period may be only 2-3 weeks if an all-inclusive testing strategy is used. Most antibody tests currently on the market have near perfect and equivalent degrees of sensitivity for detecting most individuals who are infected with HIV (epidemiologic sensitivity), but they vary in their ability to detect low levels of antibody (analytical sensitivity), such as those occurring before complete seroconversion.(2) Although tests are available to detect specific HIV immunoglobulin M (IgM) antibody, these tests have shown little utility in identifying early infection because IgM responses to HIV are not produced consistently during early infection.(4) The ability of some tests (eg, third-generation tests) to detect IgM antibody simultaneously with immunoglobulin G (IgG) detection, however, may be responsible for their higher analytical sensitivity.

Tests to Screen for HIV Infection

For the laboratory diagnosis of HIV, the mere presence of specific antibodies signals that infection has occurred. For the diagnosis to be correct, however, detection depends on the use of tests that are effective in identifying HIV antibodies, and not antibodies directed to other infectious agents that may be antigenically similar. Antigens used in HIV diagnostic tests must be appropriately specific, and usually are purified antigens from viral lysates, or antigens produced through recombinant or synthetic peptide technology. The use of such antigens allows HIV screening tests to possess both sensitivity (to detect infection) and specificity (to detect noninfection). In the United States, screening tests for HIV must be licensed by the Food and Drug Administration (FDA), regardless of whether they are used for screening blood, diagnosis, or monitoring disease.

Considerations in Choosing a Screening Test Methodology

Regardless of the particular screening test used, serum or plasma samples first are tested (screened) using a test with high sensitivity, most often an enzyme-linked immunosorbent assay (ELISA), "rapid test," or "simple method" (described below). ELISA is the screening method used most commonly, with the other 2 methodologies offering more rapid results with simple procedures applicable for use in point-of-care testing and in developing countries. With the advent of new therapies to treat HIV infection and the recommendation to institute therapy as soon as possible (but no later than 72 hours) after exposure,(5) rapid assays may be the most appropriate for testing the source patient after exposure (eg, needlestick injuries). More recently, tests have been developed using fluids that can be obtained conveniently outside the clinical laboratory. Whole blood from fingerstick and oral fluid (saliva) has been shown to be as effective as serum or plasma for detecting antibodies to HIV.

Reactive Results

Regardless of the screening method, a sample producing a reactive result must be screened again in duplicate, with at least 2 of the 3 results being repeatedly reactive before verifying infection with confirmatory assays. The most common reason for nonrepeatable results by screening tests is technical error.

Samples that produce repeatedly reactive results by screening tests must be further tested using confirmatory tests, or other confirmatory strategies (see below). Although screening tests are exquisitely sensitive, they lack an adequate degree of specificity. An example is their low predictive values when testing a population having a low prevalence of infection. When testing a population of 100 individuals, a test having a specificity of 99% can be expected to produce 1 false-positive result. If 1 individual in that same population is truly infected, the test will produce 2 positive results (1 from the infected individual, and 1 false positive). Therefore, if a positive result is produced when testing these 100 individuals, there is only a 50% chance that it represents an accurate result. Consequently, additional testing is required to differentiate between true- and false-positive results. A complete review of screening assays and a description of the use of test indexes has been published.(1)

HIV Screening Assays

Enzyme-Linked Immunosorbent Assays/Enzyme Immunoassays

ELISA is the most commonly used type of test to screen for HIV infection because of its relatively simple methodology, inherent high sensitivity, and suitability for testing large numbers of samples, particularly in blood testing centers. More than 40 different ELISA test kits are available, but only about 10 are licensed by the FDA for use in the United States.(1)

A common feature of all varieties of ELISA is the use of enzyme conjugates that bind to specific HIV antibody, and substrates/chromogens that produce color in a reaction catalyzed by the bound enzyme conjugate. The most popular ELISA involves an indirect method in which HIV antigen is attached to a well of a 96-well microtiter plate. Antibody in the sample is allowed to react with the antigen-coated solid support, usually for 30 minutes at 37º C or 40º C. After a wash step to remove unbound serum components, addition of a conjugate (an antihuman immunoglobulin with a bound enzyme) binds to the specific antibody that is attached to the antigens on the solid phase. Following another wash, addition of an appropriate substrate results in color development that is detected by a spectrophotometer and is proportional to specific HIV antibody concentration in the sample. Optical density (OD) values are produced as the colored solution absorbs transmitted light, and provide an indication of the amount of color, which is proportional to the amount of antibody bound (ie, antibody concentration). A mathematical calculation, usually based on the OD of the negative controls multiplied by a factor, produces a cutoff value on which the OD of the sample is compared to determine the antibody status; samples with OD cutoff values >1.0 (in an indirect ELISA) are considered antibody reactive (positive). Several indirect ELISA tests incorporate polyvalent conjugates (anti-IgG and anti-IgM) and antigen-sandwich configurations in order to increase sensitivity for detecting early infection (during seroconversion).

Alternate ELISA methodologies include a competitive format in which specific HIV antibody in the sample competes with an enzyme-bound antibody reagent for antigen sites on the solid phase. In this method, color development is inversely proportional to specific HIV antibody concentration.

A more recent addition to ELISA technology is the antigen sandwich method in which an enzyme (alkaline phosphatase or horseradish peroxidase) is conjugated to an HIV antigen (similar to the immobilized antigen on the solid phase). The antibody in the sample is "sandwiched" between 2 antigen molecules, 1 immobilized on the solid phase and 1 containing the enzyme. Subsequently, the addition of substrate results in color development in proportion to antibody concentration. The antigen sandwich ELISA is considered the most sensitive screening method, given its ability to detect all isotypes of antibody (including IgM).(2) One disadvantage of this method is the relatively large volume (150 µL) of sample required, which may make repeat testing and testing of samples from infants difficult.

Antibody Testing Strategies for Identifying Early HIV Infection and Estimating Incidence: Sensitive/Less-Sensitive ("Detuned") Assays

In contrast to the prevalence of HIV infection (ie, the number of persons infected), the incidence of HIV infection is defined as the change in prevalence of infection over time (ie, the number of new infections occurring). Incidence estimates most often are calculated by testing a cohort of individuals at 2 different time periods and observing the number of new infections. As is easily understood, this strategy is difficult due to the need to locate individuals for follow-up testing. However, incidence estimates are important, not only for identifying specific populations where educational endeavors can have the most benefit or where changes in infection patterns are occurring, but also for targeting these populations for therapeutic intervention or vaccines.

Detection of individuals in early infection provides several benefits. Identifying infections within the previous 4 months facilitates tracking of intravenous drug and/or sexual contacts, as only contacts in a defined, recent time period require tracing. Further, because the high viral loads of early infection are associated with increased transmission risk, identification of high-incidence populations may assist in effectively targeting prevention interventions. Monitoring areas of high incidence of HIV infection has clinical and therapeutic implications for neonatal diagnosis and for the early initiation of antiretroviral treatment, and also can provide information for prognosis, identify communities most likely to benefit from preventive vaccines, and assist in the enrollment of recently infected individuals in studies of pathophysiology or pharmacotherapy.

Laboratory-based strategies that can distinguish recently infected individuals from those with established infection have been devised. In these methods, the procedures of conventional ELISA or rapid assays have been modified to allow discrimination of antibody titer or antibody avidity. These modified assays have been called "detuned" assays or "sensitive/less-sensitive" (S/LS) assays.

During acute HIV infection, prior to the appearance of antibody (window period or pre-seroconversion), HIV infection can be confirmed only by the demonstration of circulating p24 antigen, or by the presence of viral RNA or DNA. Although highly sensitive antibody assays exist to detect very low levels of HIV antibody in blood, the window period prior to appearance of antibody rarely can be shortened to less than 3 weeks. Once antibody has appeared, titers progressively increase during 3-5 months until levels peak, at which time they remain fairly constant throughout the remainder of infection. Also, antibodies during early infection usually are of low avidity, but avidity increases as infection progresses. Therefore, HIV infection can be divided into categories of recent or established infection, depending on the quantity of antibody present or their avidities. These parameters can be exploited as tools in order to estimate the relative time that HIV infection occurred. For example, if antibody titers or antibody avidity is low, it is likely that infection occurred within the past 4 months; conversely, high-titer or high-avidity antibodies signal an established infection that has been present for longer than 4 months. Several epidemiological studies have used the S/LS testing strategy to predict incidence in San Francisco and in Rio de Janeiro, Brazil.(6-8)

The first S/LS strategy, then, is based on the principle that antibody titer increases with time and that recent infection can be assumed if test results become nonreactive following dilution of the individual's serum. In such a case, an initially reactive sample, when tested with the routine (sensitive), becomes nonreactive when diluted in a modified (less-sensitive) assay. Conversely, the serum of an individual with established HIV infection would remain reactive following dilution in the less-sensitive assay due to high levels of antibody. This strategy is used only on individuals who are confirmed positive using the Centers for Disease Control and Prevention (CDC) interpretive criteria via Western blot, as persons who are negative for antibody would not be candidates for determining the time of infection. This system, also known as the Serologic Testing Algorithm for Determining Recent HIV Seroconversion (STARHS), has been developed and adopted by the CDC. The initial assay system that was modified by dilution and validated using persons of known seroconversion or infection times was the FDA-licensed, first-generation Abbott HIV-1 Viral Lysate ELISA (3A11).(9) Specific modifications in the procedure of the 3A11 ELISA were made to 4 parameters in order to decrease the sensitivity (for the less-sensitive assay). To construct the less-sensitive test, the sample dilution was increased to 1:20,000, the sample incubation time was reduced to 30 minutes, the conjugate incubation time was reduced to 30 minutes, and the OD cutoff value was adjusted. In order to compare results obtained with the less-sensitive assay, OD readings for individual samples are standardized by calculating standardized OD (SOD) values based on the formula: SOD = (sample OD value - negative control OD value)/positive control OD value). A cutoff SOD (0.75) has been determined statistically and nonreactive samples have an SOD less than the cutoff. When such a sample shows this reversion by the S/LS test, the time interval from seroconversion was calculated to be 129 days or about 4 months (95% confidence interval: 109-149 days). Several studies have validated the S/LS algorithm by analyzing individuals with known early infection as determined by clinical evaluation, recent seroconversion, high-risk behavior, and antigen and nucleic acid analyses. A limitation of the S/LS test strategy may be the detection of individuals with long-standing infection (0.4%) and late-stage AIDS (2%). Thus, CD4 cell counts and clinical information may be required to support results obtained by the S/LS test algorithm for maximum accuracy. The S/LS strategy is inexpensive, reproducible, and can give a fairly accurate estimate of the time of infection. Because the 3A11 ELISA no longer is available, other test kits (Vironostika, bioMérieux) have been substituted, and have been considered to be equivalent in performance. More recently, another quantitative ELISA method has been introduced, and reportedly performs effectively with samples from persons who are infected with non-B HIV clades. This assay, the BED assay (Calypte; Lake Oswego, OR), incorporates synthetic peptide antigens and can classify infections for clades B, E, C, and A/D.(10)

The second method to identify the time of infection for incidence estimation is based on antibody avidity and has been developed using a third-generation ELISA. This method is known as the Avidity Index Protocol. Avidity describes the collective interactions between antibodies and a multivalent antigen. Avidity measurements are used with a variety of infectious diseases to offer confirmatory evidence of acute infection, to distinguish reactivation from primary infections, and to permit diagnosis of acute infection from a single sample. An individual's differential binding or avidity index (AI) correlates with the estimated length of time from the initial infection by HIV. Thus, the strength of the interaction between antigen and the antibody present in early infection is weak because low-avidity HIV-1 antibody comprises the majority of antibodies found in early infection. The relative avidity of antibody is stronger in established infection and can be estimated serologically based on resistance of the antigen-antibody complex to chaotropic agents. Chaotropic agents are dissociating reagents such as urea (at concentration of 4, 6, and 8 M), potassium thiocyanate (KSCN; 1-3 M), magnesium chloride (2 and 4 M), diethylamine (0.025, 0.05, and 0.1 M), and guanidine HCl (3 and 6 M).

The most widely recognized AI test is a recombinant viral lysate enzyme immunoassay (EIA) from Bio-Rad Laboratories (Hercules, CA), modified by the incorporation of a dissociation and wash step. The chaotropic agent that demonstrated the ability to dissociate low avidity HIV antibody molecules most effectively was 2.5 M KSCN. Procedurally, duplicate wells of a diluted sample are incubated with HIV antigen. Antibodies to HIV bind to the antigen, and following a wash step, a solution of dissociating reagent is added to one of the wells (test) while wash solution is added to the other well (control). Results are interpreted based on a calculation of the AI from a percentage of the ratio of the OD of the KSCN-treated specimen to that of the nontreated control. Samples demonstrating an AI <80% are taken to represent early infection and are associated with the 3- to 4-month (120-day) time period following seroconversion. This method has been validated with samples from seroconversion panels and samples from individuals with clinically established HIV infection.

Our laboratory has developed a rapid S/LS method using the Uni-Gold HIV test (Trinity Biotech; Wicklow, Ireland), a 10-minute, visually read, rapid test. This method, based on a dilution of serum for the LS mode, has shown excellent results in comparison with the Abbott 3A11 assay and when assessed using samples from individuals with known seroconversion dates. In addition, we obtained preliminary results using an HIV saliva test, SalivaCard (Trinity Biotech), that shows utility as an S/LS tool.(11) More recently, we have developed a simple and low-cost particle agglutination assay as an S/LS assay and shown it to be 97% accurate (unpublished observation). The advantage of rapid and simple S/LS assays is that they are portable and can be used to identify high-incidence populations in remote areas where ELISA instrumentation cannot be supported. Further, even in developed countries, they can be adapted easily for use in mobile testing centers to identify recently infected individuals so that they can be counseled appropriately to find contact persons within the past several months or to immediately direct individuals to appropriate treatment centers. Finally, the noninvasiveness of saliva-based rapid assays may increase testing participation.

Fourth-Generation Assays for the Simultaneous Detection of HIV Antigen and Antibody

Antibody can be detected in a majority of individuals within 6-12 weeks after infection using the earlier generations of assays, but may be detected within 3-4 weeks when using the newer third-generation antigen sandwich assays.(2) The window period can be shortened to about 2 weeks using p24 antigen assays or reduced to 1 week with the implementation of nucleic acid detection assays.(12) Consequently, the window period between infection and detection of infection may be <2 weeks if a comprehensive testing approach is utilized. The detection of p24 antigen by ELISA is a simple cost-effective technique to demonstrate viral capsid (core) p24 protein in blood during acute infection due to the initial burst of virus replication after infection. In order to maximize the detection of all infected individuals, including those in early infection, antibody, antigen, and viral RNA tests should be used. However, viral RNA tests are expensive, time consuming, and unavailable in many laboratories. Laboratories that possess ELISA capability can increase the ability to detect most infections by testing for both HIV antibody and p24 antigen. During the late 1990s, assays in an ELISA format that have the capability to detect both HIV antibody and HIV p24 antigen simultaneously were developed, thereby eliminating the need to perform separate assays.

The new generation of combination ELISAs that simultaneously detect both antigen and antibody has been developed and marketed, and offers advantages for decreasing the time, personnel, and costs necessary to perform each assay individually. These assays have demonstrated a high analytical sensitivity of detection that is most likely attributed to the combination of a third-generation format (antigen sandwich) for antibody detection and the ability to simultaneously detect HIV p24 antigen. To date, there are 8 commercial, combination antibody and antigen assays that have been developed and evaluated.(13-31) These fourth-generation assays include the VIDAS HIV DUO Ultra (bioMérieux; Marcy l'Etoile, France), Enzymun-Test-HIV-Combi (Boehringer; Mannheim, Germany), Vironostika HIV Uni-Form II Ag/AB (Organon Teknika; Boxtel, Netherlands), AxSYM-HIV Ag/AB (Abbott Laboratories; Abbott Park, IL), Enzygnost HIV Integral (Dade Behring; Marburg, Germany), Genescreen Plus HIV Ag-AB (Bio-Rad), and COBAS Core HIV Combi (Roche Diagnostics; Mannheim, Germany). The eighth assay is an 18-minute, double-antigen sandwich combination assay called the Elecsys-HIV Combi (Boehringer) that has been reported to have a specificity of 99.8% when challenged with a cohort of hospitalized patients.(16) This rapid assay is based on electrochemiluminescence and is reported to reduce the window period by 5 days over antibody tests. A ninth, unidentified assay is a lineal immunoenzymatic assay evaluated to have a sensitivity of only 99.5% and a specificity of 94.8%.

The benefits of testing for both antibody and antigen are justifiable due to the need to identify individuals with both established and early HIV infection not only for the blood donor population but also for some clinical applications. Early detection of infection via antigen testing promotes the prompt referral of infected individuals for the initiation of treatment, counseling, and prevention interventions to reduce the risk of transmission. Due to their ability to detect p24 antigen, the fourth-generation ELISAs will be of value in detecting early infection. These assays are highly applicable for the diagnosis of early and established HIV infection by hospital and private clinical laboratories and other laboratory settings. In these settings, individuals to be screened for infection are of higher risk groups than the blood donor population, and thus require the use of testing methodologies with high levels of analytical sensitivity to detect primary infection. Of significance, the high level of analytical and epidemiological sensitivity demonstrated by most of the fourth-generation assays with seroconversion and clade panels, as well as a variety of patient populations, makes them ideal for use in a variety of testing situations for the diagnosis of early and established infection. In routine laboratory settings, HIV-infected samples that are identified via antigen detection would not have been identified by the usual screening antibody assays, because antigen testing of patients is not performed commonly as a screening tool outside blood banks. The detection of early infection has been shown to be beneficial for the prompt initiation of appropriate antiretroviral therapy in a clinically relevant time frame. Additionally, early detection will help in the timely implementation of interventions such as the counseling of patients, prevention of transmission, and management of infection.

Rapid Tests

Rapid assays for detecting specific HIV antibody were developed in the late 1980s, and are defined as tests that can yield results in <30 minutes. These tests gained popularity in the early 1990s, and as technology became refined, proved to be as accurate as the ELISA when performed carefully by experienced personnel. Technical errors are common with these assays, however, because users become careless with these simple procedures. For example, pipettes are not always held in a vertical position as recommended, resulting in an incorrect delivery of reagent volumes. In addition, many laboratory workers attempt to test multiple samples simultaneously, resulting in inaccuracies in the timing of steps.

When performed correctly, rapid HIV assays are accurate and have wide utility in a number of testing situations. Application includes emergency rooms, physicians' offices, point-of-care testing, autopsy rooms, funeral homes, small blood banks, and situations involving stat HIV testing (where immediate treatment is recommended for exposures). Rapid HIV assays have proven particularly useful for testing pregnant women in labor who have not received prenatal care (ie, of unknown HIV status). It has been shown that the institution of antiretroviral therapy (eg, zidovudine) is effective in reducing transmission of HIV, and that this should be provided as early as possible to the mother and subsequently to the newborn. Rapid HIV testing of the mother who is near delivery allows treatment to be initiated prior to delivery if a positive serostatus is determined.(32) Importantly, these rapid assays are easy to perform and have utility in developing countries, where facilities may not be optimal, stable electricity may be unavailable, and formal education programs for laboratorians are absent.

One class of rapid tests is the "dot blot" or "immunoblot"; they produce a well-circumscribed colored dot on the solid phase surface if the test is positive. Most of these rapid assays now incorporate a built-in control to indicate that the test was performed correctly. This control is an antihuman immunoglobulin that binds any immunoglobulin in the sample and produces a separate indicator when all reagents are added appropriately. In addition, several varieties are available that include 2 "dots," which allows the differentiation of HIV-1 and HIV-2 infection. The procedures for the dot-blot assays are similar regardless of the exact format of the test. Most require drop-wise additions of reagents in the following sequence: buffer, sample, wash buffer, conjugate, wash buffer, substrate, and stop solution. Some assays substitute an IgG binding dye (protein A gold reagent) for the antiimmunoglobulin conjugate, thereby decreasing the procedure by a step.

The newer 1-step rapid assays, also known as immunochromatographic assays, are convenient, self-contained tools for HIV serologic testing, consisting of a flat cartridge device, usually plastic or paper. Whole blood, oral fluid, or serum is placed at the tip of the device and allowed to diffuse along a strip that is impregnated with reagents (often protein A colloidal gold) that bind and permit visual detection of HIV antibodies; some use third-generation (antigen sandwich) technology. These tests can be completed in <10 minutes (some within 2 minutes), require little or no addition of reagents, and contain a built-in quality-control reagent to control for technical errors. Some tests can be stored at a wide range of temperatures (from 15º C to 30º C), and are transported easily. For example, one type (Determine; Abbott) comes in "cards" of 10 tests each, making it possible to carry 100 tests in a shirt pocket; the cards require no reagents, just addition of serum or plasma. The test can also be performed on whole blood, or blood collected via fingerstick (this requires 1 buffer addition). These types of rapid HIV tests are gaining in popularity because of their simplicity, ease of interpretation, and robustness.(33,34) In particular, the fingerstick collection method is taught easily to health care personnel in outreach situations or mobile vans. The use of fingerstick specimens also may prevent unnecessary collection and discarding of full units of donated blood (where blood is collected prior to testing at a remote laboratory and held until results become available). Another variety of lateral flow devices allows for the use of saliva, plasma, whole blood, or fingerstick specimens, thereby adding flexibility in sample type (see "Alternatives to Classic Tests and Testing Strategies" below).

Other rapid test formats include dipsticks, in which antigen is attached on the "teeth" of comblike devices; several of these rapid tests have the ability to differentiate HIV-1 and HIV-2. Disadvantages include a subjective interpretation, difficulty in reading if the laboratorian is color-blind, and a higher cost than that of the ELISA. Currently, 4 rapid HIV tests are approved for use in the United States.

Simple Tests

This type of HIV test requires longer than 30 minutes for results, but consists of procedures that can be performed easily without instrumentation. Within this class of tests are agglutination assays in which antigen-coated particles (red blood cells, latex particles, or gelatin particles) are allowed to react with serum antibodies to form visible clumping (agglutination). If red blood cells are used, the technique is termed passive hemagglutination; with the use of latex particles, it is known as latex agglutination. In East Asia, an HIV gelatin particle agglutination test is popular, offering good sensitivity, low cost, and ease of performance. It incorporates a quality control system to detect nonspecific antibodies directed toward the gelatin particles themselves, and results can be obtained within 2 hours with minimal hands-on time. Although appropriate for use in facilities with limited testing capabilities, this test must be performed under temperature-controlled conditions.

Tests to Confirm HIV Infection

Most testing algorithms require the use of very specific assays, such as the Western blot, indirect fluorescent antibody (IFA) assay, or the radioimmunoprecipitation assay (RIPA), to verify reactive screening test results. If performed and interpreted correctly, these extremely specific tests should not produce biologic false-positive results. They are, however, more laborious and more expensive than screening assays.

These confirmatory tests do not have to be FDA licensed in the United States when used for purposes other than testing blood donors. For blood donors, a licensed confirmatory test is used for purposes of donor reentry, for which the results must be negative. The primary purpose of confirmatory tests is to ensure that uninfected individuals who test reactive by screening assays are not identified incorrectly as being HIV infected.

Western Blot Test

Methodology

The Western blot probably is the most widely accepted confirmatory assay for the detection of antibodies to the retroviruses. Most authorities consider it the gold standard for validation of HIV results. It is based on using an electrophoretic technique to separate HIV antigens derived from a lysate of virus grown in culture. This technique denatures the viral components, imparts a negative charge to the antigens, and separates them primarily on the basis of their molecular weights. The separation of antigens in the technique allows for the identification of specific antibodies to each of the viral antigens in a subsequent set of steps similar to the ELISA methodology.

A purified HIV antigen mixture is layered onto a sodium dodecyl sulphate (SDS) polyacrylamide gel slab and then electrophoresed. The viral proteins (HIV antigens) migrate through the molecular pores of the gel at rates determined by electrical charge and molecular weight. The proteins with higher molecular weight migrate less and form bands closer to the starting point. The proteins on the gel are then transferred ("blotted") to nitrocellulose paper by another electrophoretic procedure. This paper is cut into thin strips, each with the full distribution of viral protein antigen bands. A single test strip is incubated with a 1:50 or 1:100 dilution of a test sample or a control and then washed and incubated with a labeled (tagged) antihuman globulin. At this point, the procedure is similar to any other indirect immunoassay. The label usually is an enzyme (horseradish peroxidase or alkaline phosphatase) that will react with a specific colorless substrate to produce an insoluble colored band on the strip wherever there is an antigen-antibody complex. Reaction with a positive serum sample produces a pattern of bands on the strip that is characteristic of HIV. Many of these bands have been identified as specific viral gene products.

The HIV-1 viral antigens are separated as follows (from top to bottom): gp160, gp120, p66, p55, p51, gp41, p31, p24, p17, and p15 (Figure 1). The "gp" designation refers to glycoproteins; "p" indicates proteins. The numeric values (x100) indicate molecular weights. It is important to remember that nonviral proteins derived from the host cells in which the virus was grown also are present on the nitrocellulose strip. They can form bands in many places, but often are near the middle molecular weight (40,000 to 60,000) region. These nonviral protein bands may produce difficulty in interpretation of results by producing nonspecific reactions.

Interpretation of Results

Depending on the particular antibodies in the sample, reactivities with the separated antigenic components result in band profiles. The type of profile (the combination and intensity of bands that are present) determines whether the individual is considered positive for antibodies to HIV. The classification of Western blot results is determined by certain criteria. Most institutions now follow the CDC guidelines, which require reactivity to at least 2 of the following antigens: p24, gp41, gp120/160 for a positive classification. It is now universally accepted that a negative result is the absence of all bands. Two organizations, however, including the World Health Organization (WHO), suggest that results also can be reported as negative if there is only a very weak p17 band. Indeterminate classifications occur when there is reactivity to 1 or more antigens, but not fulfilling the criteria for positivity. Figure 1 depicts examples of positive, negative, and indeterminate Western blot results.

Unfortunately, sera from some noninfected individuals show some reactivity to 1 or more antigens if tested by Western blot. This reactivity may occur in as many as 15% of normal noninfected persons, and many times occurs in persons who are nonreactive by screening assays. Therefore, if ELISA-nonreactive sera are tested by Western blot, many will result in an indeterminate profile. Most indeterminate results show only weak reactions to the Gag proteins (mostly p17, p24 and/or p55); other patterns occur but are less frequent. Any Western blot reactivity that does not meet the requirements for being positive or negative must be considered indeterminate.

Some individuals who exhibit indeterminate results (eg, reactivity to p24 and p55) later seroconvert, demonstrating that a p24 and p55 profile can indicate early infection. Conversely, other individuals may have the identical profile for long periods of time (years) and never seroconvert (ie, they are not infected). In fact, most indeterminate Western blot results from noninfected individuals exhibit the p24 and/or p55 profile. Therefore, an indeterminate Western blot result cannot predict early infection.

Most authorities suggest that persons with indeterminate results should be retested after several months, although seroconversion may be detected in a shorter period of time. If at all possible, the retesting of an individual at a later time should be performed in parallel with reassay of the initial sample on the same run with the same kit lot numbers and the same assay conditions to ensure that the samples can be compared directly. The WHO recommends retesting persons after 2 weeks if highly suggestive Western blot profiles are produced, although other organizations suggest waiting 1-6 months before retesting. If an individual is retested over a period of 6 months and becomes negative or the band profiles do not progress, infection with HIV generally can be ruled out. For poorly understood reasons, many individuals continue to exhibit indeterminate results for years but are not infected. If an individual does progress serologically (more bands or greater intensity of bands) or converts to positive (seroconversion) during retesting, the individual probably was infected at the time of the first test (early infection). It should be noted that individuals who have received vaccination for HIV (eg, subunit gp160) may be misidentified as positive based on reactions to the envelope antigens alone.

The significance of an indeterminate Western blot result varies depending on the risk factors, clinical status of the patient, and the Western blot profile produced. For example, individuals with a history of high-risk behavior are more likely to be the ones who later seroconvert, because the chances of their being infected are high. In addition, some Western blot profiles are more suggestive of early infection (eg, p24, p31, and p55) than are others (eg, p17 only). Many initially indeterminate results that subsequently become negative or remain indeterminate probably are a result of nonspecific reactions, hypergammaglobulinemia, the presence of cross-reactive antibodies, infection by HIV-2, or infection by an unknown, but related retrovirus. There have been a few reports where autoimmune diseases (eg, systemic lupus erythematosus) can cause false-positive HIV tests, including Western blot.(35) Also, it is known that some individuals with AIDS may lose reactivity to p24, and perhaps other antibodies, later in disease, so that even AIDS patients may have indeterminate Western blot results by some criteria. Ancillary tests, such as polymerase chain reaction (PCR) and viral culture may be helpful in resolving these indeterminate results if the diagnosis is in question.

Indirect Immunofluorescent Antibody Assay

In this technique, cells (usually lymphocytes) are infected with HIV and are fixed to a microscope slide. Serum containing HIV antibodies is added and reacts with the intracellular HIV. The slide is washed and then allowed to react with antiimmunoglobulin antibodies with a covalently bound fluorescence label attached. The reaction is visualized using a fluorescent microscope. This technique has the advantage of sometimes providing definitive diagnosis of samples that have yielded indeterminate results by Western blot analysis. Disadvantages to its use include the requirement of an expensive microscope and a subjective interpretation, thus necessitating well-trained individuals.

Modified Western Blot

Western blot assays that have the ability to identify and differentiate infections by HIV-1 and HIV-2 have been developed. Most incorporate the use of viral lysates from HIV-1 and synthetic peptides artificially applied from HIV-2 on the same nitrocellulose strip (a modified or augmented Western blot). In this case, multiple HIV-1 antigens and 1 HIV-2-specific band (gp36 or gp41) are present on the strip. Criteria established by manufacturers include reactions to 1 gene product from each of the 3 major groups (Gag, Pol, and Env) for positivity for HIV-1. To be considered positive for HIV-2, the test must show reactions to the HIV-2-specific antigen plus a reaction to HIV-1-specific antigens, which alone do not meet the criteria for positivity for HIV-1.

Line Immunoassay

Another alternative to the classic Western blot and IFA confirmatory tests is the line immunoassay (LIA). In this assay, recombinant or synthetic peptide antigens are applied on a nitrocellulose strip, rather than electrophoresed as in the Western blot. This use of "artificial" antigens decreases the presence of contaminating substances derived from cell culture that can cause interference and sometimes false reactions. The use of LIA is popular in Europe, but these tests have not been licensed for use in the United States. A number of reports have verified that the accuracy is equivalent to the Western blot.

HIV-2 Tests

HIV-2 is endemic primarily in areas of West Africa, but the increased prevalence and distribution of HIV-2 infections necessitate the use of tests that can detect HIV-2 infection. In the United States, about 80 cases of HIV-2 have been verified, most of which have been linked to West Africa. Biologically, HIV-2 is very similar to HIV-1: the HIV-2 genome exhibits about 60% homology in the more conserved gag and pol genes, and 30 to 40% homology in the other viral genes and long terminal repeat (LTR) sequences. The antigens of HIV-2 are similar to those of HIV-1, but the molecular weights may vary slightly. For example: the Gag proteins of HIV-2 have designations of p56, p26, and p16; the Pol proteins p68 and p34; and the envelope glycoproteins gp36 (or gp41), gp140, and gp105. As with HIV-1 screening tests, a variety of test formats are available to detect antibodies to HIV-2, including ELISA beads, ELISA microtiter, and rapid/simple assays.

Diagnostically, HIV-2 infections can present problems. Screening tests designed to detect infection by HIV-1 do not always detect infection by HIV-2 and vice versa. Most cross reactions represent antibody induced by the core (p26) and/or Pol antigens (p68, p34), because these are highly conserved between the two different viruses. A lack of reactivity with heterologous viruses, however, dictates the need for an extra measure of vigilance to identify infections that might not be readily apparent using some HIV-1 assays. By HIV-1 ELISA, the OD readings of HIV-2-positive specimens may be high negative; by Western blot, the results may be indeterminate. Therefore, it is important to recognize slightly high negative readings and suggestive indeterminate results by HIV-1 tests, and consider evaluating the serum using HIV-2 tests.

To address this issue, commercially available HIV-1/2 "combination tests," which incorporate antigens from both viruses, can be used to screen sera in an attempt to identify either infection. The subsequent differentiation of HIV-1 and HIV-2 infections necessitates the use of highly specific ELISA (eg, synthetic peptide-based), Western blot, radio-immunoprecipitation assays, or PCR.

In late 1991, the FDA licensed the first combination HIV-1/HIV-2 screening test and recommended that blood banks start screening for HIV-2 by mid-1992. Blood banks in the United States can use either the licensed HIV-2 ELISA screening test together with the HIV-1 ELISA, or one of the licensed HIV-1/2 combination tests. Samples that test positive by the combination test are tested by an HIV-1 Western blot. If the result is negative or indeterminate by this HIV-1 Western blot, 1 or more specific HIV-2 tests are used to further analyze the sample. Combination tests are considered to be equivalent to their predecessors in terms of sensitivity (ie, near perfect).

HIV-2 confirmatory tests include the Western blot and the RIPA. In addition, EIA tests and some rapid tests that use chemically synthesized peptides corresponding to a unique immunogenic region within the respective transmembrane glycoproteins exhibit good correlation with the Western blot and the RIPA for identifying and differentiating HIV-1 and HIV-2 antibodies. Furthermore, these tests are valuable for differentiating samples that produce reactions to both viruses (dual reactors).

For HIV-2 confirmation, most organizations that have created criteria for positive HIV-2 Western blot agree on the necessity for reactivity to the envelope antigens. The WHO requires reactivity to at least 2 HIV-2 envelope antigens, whereas other organizations require reactivity to p26 (Gag) and to gp34 or gp105 (Env). If a specimen is tested by both HIV-1 and HIV-2 Western blot, the blot exhibiting the strongest reactivity to envelope antigens usually indicates which infection is present.

Alternatives to Classic Tests and Testing Strategies

As technology evolves, alternatives to the classic tests and testing strategies arise. Each offers 1 or more attractive features that may simplify collection, testing, or interpretation of results.

Oral Fluid ("Saliva") HIV Tests

Noninvasively collected specimens, such as oral fluids, have been used for HIV testing as a more convenient alternative to blood samples. Although generally referred to as "saliva," the fluid used for testing is actually crevicular fluid from capillaries beneath the tooth-gum margin, which is a transudate of blood and therefore similar to the samples used in serum-based tests. The concentration of antibodies in oral fluids is about 1/400 of that in plasma, however, because of the dilutional effect of fluids from the salivary glands (true saliva),(36) necessitating extremely sensitive tests that are able to detect small quantities of antibody. The testing technology to detect these low quantities is now available, and oral fluid tests, both ELISA and rapid tests, are accurate.(37,38)

A complete review of oral fluid testing for HIV has been published.(36) The use of oral fluids for testing offers advantages, such as ease of collection, group collections, collection from persons in whom blood is difficult to obtain, and an increase in collection adherence.(1,36)

Rapid oral HIV tests were introduced in the mid-1990s.(37) As with ELISA, the sensitivity of these tests to detect HIV in oral fluid needed to be boosted because of the low level of antibody in oral fluid, which was compounded by the dilutional effect of pure saliva.(36) In 2004, a rapid HIV test was licensed by the FDA for use with oral fluid. This test, the OraQuick Advance (OraSure Technologies; Bethlehem, PA), is a combination collection and testing device. Consisting of an absorbent (porous) pad on a stick coupled to a lateral flow testing device, it is swabbed once around the gums, and then placed in a vial of buffer solution. Following a 20-minute incubation, the results are read like other lateral flow rapid tests (a control line is included also). The manufacturer claims 100% sensitivity and specificity equivalent to that of ELISA HIV tests. Therefore, a positive result must be considered preliminary until confirmed by a more specific test, such as Western blot. This device also can be used for testing plasma, whole venipuncture blood, or blood collected via fingerstick, thereby giving flexibility for different testing situations. As of March 2006, rapid oral HIV testing is approved for use only by clinical laboratories and Clinical Laboratory Improvement Amendments-waived laboratories, but licensing for home use remains under consideration.

Urine Tests

Intact IgG antibodies are found in urine, but their exact origin is unknown. The collection of urine is simple, noninvasive, and inexpensive, and the sample can be stored at room temperature for extended periods of time. The use of urine for testing is appropriate for physicians' offices, health clinics, and in developing countries where health care personnel may not be trained professionally or where clean needles for drawing blood may not be available. The major disadvantage is that there is not an approved urine-based confirmatory assay, necessitating the collection of blood when results are reactive. The FDA has approved an ELISA and Western blot for use to test urine for antibodies to HIV-1.

Although urine testing for HIV has not gained in popularity as much as would be expected, companies are interested in modifying their serum-based rapid assays to offer rapid tests that can use urine samples. The market for these tests is the same as that for rapid serum tests, ie, occupational exposure cases, pregnant women without known HIV status, and public health clinics that provide results during the initial visit (to prevent loss to follow-up when patients do not return for their results). Although it would seem that serum-based tests could be modified easily to accept urine samples, this is not the case. There are a number of factors that influence rapid tests differently from the way they influence ELISA-type tests. For example, because urine is much less viscous and contains less protein than serum, flow rates through these rapid devices are increased dramatically. Consequently, this leaves less time for antigen-antibody reactions to occur. Also, the variability in the pH of urine appears to affect reaction time (since antigen-antibody reactions are pH dependent); the pH of urine varies considerably from individual to individual. However, our laboratory has been successful in modifying one manufacturer's serum-based test (only 1 of 6 manufacturers' tests could be modified successfully). Nevertheless, this shows proof of principle that rapid urine tests can be developed.

Home Collection for Testing

As of this writing, home collection, but not home testing, is approved by the FDA. These collection devices are filter paper for the collection of whole blood via fingerstick. The samples are mailed to a laboratory, eluted, and screened with ELISA tests. Results and counseling are made available by telephone. More recently, the FDA is considering the use of over-the-counter (OTC) rapid tests, particularly oral fluid tests for home use, in order to increase the prevalence of HIV testing. However, how to address needs for HIV test counseling (which traditionally includes discussion of risk reduction and education on the implications of test results) in the setting of home testing is unclear.

Alternative Confirmatory Strategies Using Screening Tests

In most industrialized countries, confirmation of HIV infection is accomplished using Western blot or IFA technologies. In developing countries, these assays may be available in reference laboratories, but it is common to find alternative confirmatory strategies for cost savings because funds to purchase expensive confirmatory tests or equipment may be unavailable. Several investigators have verified that similar predictive values can be obtained by using 2 screening assays in tandem. This method can result in up to 80% cost savings.(39) It is important to select appropriate tests, with the most sensitive tests used in the initial testing. These strategies recommend initial screening using ELISA or a rapid/simple assay, followed by a second ELISA or rapid/simple assay; the initial and second tests must be of different principle (bead vs microtiter) and/or use a different antigen source (lysate vs recombinant or synthetic peptide).

Another recent advance that makes use of prior technology, but in a novel format, includes a rapid confirmatory assay that incorporates several different HIV antigens on 1 rapid test device (similar to combination HIV-1 and HIV-2 rapid tests). These rapid, flow-through tests are performed in an identical manner to rapid screening testing (addition of several reagents in drop-wise fashion) and produce "reaction profiles" similar to those of the Western blot test and LIA. A thorough evaluation of one of these rapid confirmatory tests has produced excellent results.(40) Several companies are introducing these assays to address the issue of expensive and cumbersome Western blot confirmatory assays and the associated need for significant laboratory infrastructure.

HIV Diagnostic Dilemmas

The major difficulties with the laboratory diagnosis of HIV infections include: 1) indeterminate Western blot results; 2) minimally reactive confirmatory test results from noninfected individuals; 3) inconsistent results when repeating specimens or testing follow-up specimens; 4) the occurrence of technical errors; 5) false-negative results due to HIV Group O viruses; and 6) laboratory diagnosis of HIV infection in the newborn.

Indeterminate Western Blot Results

In reference to samples that show inconclusive results (eg, indeterminate Western blot results), a follow-up specimen in 1-3 months is the most effective means for resolution. At this interval of time, serum from almost all individuals who are infected will show an increase in reactivity by serologic assays (eg, an increase in the OD by ELISA or more bands by Western blot) or will seroconvert. It is important to test both samples on the same run to obtain a clear indication of changes in reactivity (ie, to ensure that intertest variations do not contribute to small differences in reactivity). Alternatively, IFA, PCR, viral culture, or antigen assays may be helpful.

Minimally Reactive Western Blot Results

These results occur occasionally, perhaps due to early infection (seroconversion) when antibody levels have not yet peaked, and on rare occasions for unknown reasons in individuals who are later found not to be infected with HIV. In the latter case, reactions to p24 usually are noted, as are weak reactions to gp41 or gp 120/160. In these cases, it is important to note on the report form that "on rare occasions, this profile has been found in persons who are not infected, and submission of a new specimen in several weeks is recommended."

Inconsistent Results

Inconsistent results when repeating specimens or testing new specimens from the same individual are rare, but real occurrences. Explanations include mislabeling of specimens, technical errors in the laboratory, the use of different test systems, or problems with components of the test system. If an individual is seroconverting, repeat testing by the same assay on the same specimen can produce results that fluctuate around the cutoff value. Alternatively, wide variations in values usually are a sign of technical error and should be investigated thoroughly through quality assurance monitoring. Inconsistencies with follow-up specimens can be due to seroprogression in truly infected individuals, seroreversion in persons who are not infected, or mislabeling or technical errors.

Technical Errors

Technical errors do occur, and although they cannot be eliminated totally, they can be minimized through the institution of a thorough quality assurance program and documented preventive measures. Clerical errors are the most common, and can be addressed effectively through a dedicated supervisory review mechanism. Several essential quality assurance measures are outlined subsequently.

False-Negative Results for HIV Group O

False-negative results by HIV serologic assays have been verified when testing some individuals infected by HIV Group O viruses.(41) This group of viruses, found primarily in Cameroon and Gabon, also has been reported in Europe and the United States.(42) Several "acceptable" routine HIV screening assays have been documented to produce false-negative results in up to 20% of sera from individuals infected with Group O viruses.(43) Although it is difficult to recommend measures to prevent this misdiagnosis, manufacturers of test kits are addressing this problem by incorporating antigens from Group O viruses.(44) Health care providers can be vigilant by inquiring as to the geographic origin of persons tested, or their contact with persons from these areas of Africa. The same is true for HIV-2 infections, when HIV-1-only assays are used (see above).

Diagnosis in the Newborn

The laboratory diagnosis of HIV in the neonate has been difficult since the first tests were developed, principally because of the omnipresence of maternal antibody up until 1 year after birth, at which time the infant may serorevert. Subsequently, it may be several more months until the infected infant's immune system is competent enough to produce antibody (seroconversion). Antigen assays can be of help, as can PCR, to detect HIV DNA or RNA in the infant. At present, however, definitive diagnosis in the newborn is still difficult, particularly before 6 months of age.

Monitoring the Testing Process: Quality Assurance, Quality Control, and Quality Assessment

The validity of diagnostic test results depends on the quality of a number of measures used before, during, and after the test is performed.(1) To ensure the quality of test results, a program consisting of quality assurance, quality control, and quality assessment is necessary. In most developed countries, regulatory agencies provide guidelines for such a program. In contrast, formal programs and recommendations generally are unavailable in developing countries. Brief descriptions and examples of quality assurance programs follow.

Quality assurance encompasses all measures, from receipt of specimens through final reporting, to ensure that the final results are as accurate as the assays allow. Specimens must be inspected upon arrival for suitability; logging, processing, and review of all accompanying paperwork must be performed and monitored carefully. Also included are an organized record keeping system, standard operating procedure manuals that act as references, a continuing education program, supervisory review of results, a system for evaluation of laboratory personnel, use of the most appropriate tests/strategies, a mechanism for timely reporting, compliance with regulatory requirements, storage of specimens for follow-up testing, appropriate reporting forms, variance reporting for errors/inconsistencies, a good management system, and, of course, a good quality control and quality assessment program.

Quality control refers to those specific measures that ensure the test is performing as expected. Such measures include careful inspection of internal (kit) control values that validate the test, monitoring of physical parameters (temperatures, functioning of equipment), validation of new reagents (different kit lots), and use of extraordinary measures such as external controls to verify the manufacturer's claims (the use of external controls is recommended but not required). A detailed description of quality control measures has been published.(1)

Quality assessment is a means to challenge the overall performance of the laboratory. This process usually consists of the testing of a panel of samples with known reactivity provided by an external source. Such assessment, usually performed quarterly, yields some information about the overall quality of the laboratory's performance. Results from each laboratory are compiled and feedback is provided. Other measures of assessment include internal (self-inspections of the laboratory and testing process), specimens provided by the laboratory supervisor for blinded testing by personnel, and review of the total operation by an external agency. The ultimate challenge in totally assessing the ability of a laboratory to produce accurate results is to provide these panels of specimens in a blinded manner so that personnel are unaware that they are being monitored.