Article updated and reviewed by Cyrus Badshah, MD PhD, Assistant Professor of Clinical Medicine, College of Physicians and Surgeons, Columbia University; Assistant Attending Physician, Department of Medicine, Division of Infectious Diseases & Medical Director, Chest (TB)Clinic and Directly Observed Therapy Program, Harlem Hospital Center on April 18, 2005.
HIV testing is performed to detect antibodies (proteins made by the human body) against the human immunodeficiency virus (HIV) which indicates infection with the virus.
Many HIV-infected individuals are unaware of their status, since the characteristic complications of AIDS usually do not develop until several years after HIV infection. The virus destroys immune cells known as CD4 cells or T helper cells. When the levels of CD4 cells fall below 200 cells/mm3 (generally the level is greater than 500 cells/mm3), a patient is considered to have AIDS (as opposed to HIV infection earlier in the course of the disease). With a weakened immune system, patients with AIDS are unable to defend themselves against infections a normal healthy immune system can usually ward off. Such infections are known as opportunistic infections. Early knowledge of one’s HIV status may allow infected individuals to seek treatment early enough in the course of the disease before the onset of AIDS or such opportunistic infections occur. Knowing one’s HIV status may also make an individual less likely to engage in high-risk behavior reducing the risk of HIV transmission to other uninfected individuals.
The standard method of screening for HIV infection are serologic tests which detect the presence of antibodies to HIV-1 and HIV-2 (the two -main types of HIV) in a patient’s blood. A time interval exists between infection with HIV and the development of antibodies to HIV high enough to be detectable by such tests in the blood (seroconversion). The interval between infection and seroconversion is called the “window period.” In most cases, infected individuals seroconvert within 6 months of being exposed to HIV due to high risk behavior. The importance of knowing about the window period is that an individual infected with HIV may test negative if tested during this period because there may not be sufficient levels of antibodies in the blood to be detected by the test. Therefore, individuals who test negative should remember that they may be infected despite the initial negative test result. Physicians therefore recommend that people at risk for HIV infection need to stop engaging in high-risk behaviors and return for retesting usually in six months after the initial negative test result. By six months at least 95 percent of all infected persons will have detectable levels of antibodies against HIV. An individual who tests negative at six months after possible exposure to HIV is therefore very unlikely to be infected with the virus.
HIV testing must always be preceded by pre-test and followed by post-test counseling sessions. This is done by specially trained clinicians or counselors to ensure accurate information regarding HIV testing is given to the patient in a clear, and understandable form at both sessions.
To date, HIV testing can be performed on any of three bodily fluids: blood, saliva or urine.
The HIV Antibody Blood Test is a sequence of two different types of tests. The initial test, called the ELISA test, is used to screen a large number of blood samples for the presence of HIV antibodies. A positive result on the ELISA is then confirmed by a second test.
ELISA stands for "enzyme-linked immunosorbent assay" and is pronounced like the name "Eliza." This test was initially developed to screen donated blood for the presence of HIV in order to eliminate HIV-infected blood from the blood supply. Only later was it used for screening patients for HIV infection.
A negative (nonreactive) result from the ELISA demonstrates with greater than 98% accuracy that the blood sample contains no HIV antibodies. A positive (reactive) (result from the ELISA usually means that the blood sample contains HIV antibodies (true positive). Occasionally an ELISA test may be reactive even though the sample does not contain HIV antibodies (false positive). This is the reason why an initial reactive ELISA is not immediately accepted as being a true positive. If an initial ELISA test is reactive, it is usually repeated. If ELISA test results on a sample are consistently reactive, the results are then confirmed with another laboratory test that is even more specific for the HIV antibody, (and so less likely to give false positive results). Confirmatory tests help distinguish samples that are true positives from those that are falsely positive on the ELISA.
The Western Blot Test is the most common confirmatory test used for this purpose. Some laboratories confirm ELISA-positive results with tests other than the Western blot. Alternatives include the immunofluorescence assay (IFA) and the radio-immunoprecipitative asay (RIPA). These tests are as accurate as the Western Blot Test. The rate of false positives after confirmatory testing is extremely low. Hence, a positive result on ELISA that is confirmed by Western Blot Tests, IFA or RIPA usually means (with greater than 99.7% accuracy) the blood sample has HIV anitibodies and the person has HIV infection.
Very rarely, a person with a low risk for HIV infection may get a confirmed positive result. When there are still doubts about such a result, further laboratory investigations are usually done. Generally, this involves repeating the entire sequence (ELISA followed by Western Blot, IFA or RIPA test) on a fresh sample of the subject's blood. Alternately, if available, more sophisticated testing (e.g., polymerase chain reaction, PCR) may provide a conclusive result more quickly. PCR is a process wherein a few molecules of HIV proviral DNA can be amplified into a sufficient mass of DNA to be detected by current testing methods. For example, it can determine if newborns of HIV-infected mothers are truly infected (newborn will have HIV virus in the blood detected by PCR) or if such newborns test HIV-positive because HIV antibodies have traveled from the infected mother’s blood across the placenta (newborn will have HIV antibodies in their blood), but the newborn is not truly infected (no virus in the baby’s blood as determined by the PCR test).
A newer test, the HIV Oral Fluid Test (Orasure®) was approved in 1994 and is intended for people 13 years of age and older. This testinvolves collecting oral salivarysecretions by placing a specially treated pad between the cheek and the gums and then evaluating them for the presence of HIV antibodies.
Orasure is essentially as accurate as the standard blood tests, and because it doesn't involve a needle stick, is preferred by many patients. Orasure is available through physician offices many public health clinics, community-based service organizations and AIDS service organizations.
The Urine-based HIV Test was approved in 1996 by the FDA as a screening test. The test uses urine samples to detect antibodies to HIV-1 using the ELISA method. This test can only be ordered by a physician.
In one study, the urine test was positive as an initial screening test 94 percent of the time in persons known to have AIDS. This means that the test would be expected to miss in about 6 out of every 100 asymptomatic HIV-1 infected patients. Therefore, this test is not as accurate as the blood test in detecting HIV infection.
Other studies showed that the urine-based test could give a false-positive result in one or two persons out of 100 without HIV-1 antibodies in their blood, compared to 1 in 1,000 with a blood-based ELISA test. Therefore, a positive urine ELISA screening test must be followed by a standard blood test (screening ELISA followed by a confirmatory Western Blot test or IFA or RIPA) to confirm such results.
The test results can be reported as being positive, negative or indeterminate which will be interpreted as follows:
Positive result: This means that a person has HIV antibodies detected by the test and usually means that the person tested is infected with the virus. People with a positive result should assume that they have the virus and could therefore transmit it by having their body fluids come in contact with an uninfected person as by having unprotected sex, sharing needles during drug use, donating blood, sperm, or body organs, during the birth of a newborn or by breastfeeding.
Negative result: If testing is done more than six months since a person’s last possible exposure to HIV, then a negative result strongly suggests that a person is not infected with the virus. However, while at least 95 percent of all infected persons seroconvert within six months, there still is a possibility in about 4 or 5 out of a 100 persons tested, the test results may come back falsely negative.
Inconclusive (indeterminate) result: A small percentage of results may be reported as being inconclusive or indeterminate. This means that the result is neither positive nor negative. It may be due to a number of factors that have nothing to do with HIV infection, or it may be because testing was done too early in an infection (the “window period”) when there are not enough HIV antibodies present to give a positive result. In cases of inconclusive (indeterminate) test results, another blood sample will be taken at a later time for a retesting.
As with any test, there can be false positives and false negatives. The standard blood test where an initial ELISA is followed by a confirmatory blood test is more than 98% accurate. However, a small percentage of people may be told that they have the HIV antibody when in fact, they do not (false positive). Also, a small percentage of people may also be told that they are not HIV-infected when in fact they are false negative. This can happen when the test is taken too soon after being infected and the body has not had time to produce HIV antibodies (the window period).
As discussed above, individuals who test negative should remember that there is a chance, however small, they may be infected despite the initial negative test result. In such instances, physicians therefore recommend that people stop engaging in high-risk behaviors and return for retesting in six months or maybe even earlier..
Editorial review provided by VeriMed Healthcare Network.