Screening tests for chronic HIV infection are commonly enzyme-linked immunosorbent assays (ELISAs). These tests are highly sensitive assays, positive in more than 99% of chronically infected patients.
These tests are most commonly run on serum in a hospital or laboratory setting, but home kits are available that offer anonymous testing through oral secretions, urine, or a dried blood spot. All have similar test characteristics. Later generation tests are able to accurately detect all strains of H1V-1, and false-positive test results, once feared, have become rare. However, given the history of higher rates of false-positive results and the significance of a true-positive result, confirmatory testing must always be sought if a screening ELISA is positive.
The most common reason for a falsely negative ELISA is the so-called “window period” after new infection. Since ELISA tests measure the host’s antibody response, which can take weeks to months to evolve, patients with primary HIV infection may test negative with this test modality. Older and less sensitive assays could take up to 6 months to become positive, but later generation tests are essentially universally positive within 3 months. In fact, recent data suggest that the vast majority of patients become ELISA-positive only 1 month after infection. Although false-negative results are otherwise rare, this possibility should be entertained in high-risk patients with hypogammaglobulinemia, and alternative testing may be considered.
Occasionally a provider may encounter an “indeterminate” ELISA result, and this must not be misconstrued as negative. In fact, many indeterminate results are from patients evolving primary infection, and, as reviewed, it is especially important to identify these patients. All indeterminate ELISA tests should be repeated at least once.
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December 28th, 2010 :: Filed under
HIV
Once antibodies have begun to form in reaction to the presence of HIV, a blood test known as the ELISA test may detect their presence. If sufficient antibodies are present, the ELISA test will be positive. When a person who previously tested negative (no HIV antibodies present) has a subsequent test that is positive, seroconversion is said to have occurred. In such a situation, the person would typically take another ELISA test, followed by a more expensive, more precise test known as the Western blot, to confirm the presence of HIV antibodies.
Although the ELISA is viewed as quite accurate, it is a conservative test in that it errs on the side of caution, meaning it produces a large number of false positive results. It was deliberately designed to do this because it was intended as a test for screening the nation’s blood supply. There have also been instances of false negative results. Some health professionals believe that there are chronic carriers of HIV who, for unknown reasons, continually show false negative results on both the ELISA and Western blot tests. This, of course, raises serious concerns about risks for these people’s sexual partners. It should be noted that these tests are not AIDS tests per se. Rather, they detect antibodies for the disease, indicating the presence of the HIV in the person’s system. Whether or not the person will develop AIDS depends to some extent оn the strength of the immune system. However, the vast majority of all infected people does develop some form of the disease.
As testing for HIV antibodies has been perfected, scientists have explored various ways of making it easier for individuals to be tested. Part of the reason health officials make the distinction between reported and actual cases of HIV infection is because it is believed that many people who may actually be HIV positive avoid being tested. One suspected reason is fear of knowing the truth. Another possible reason for avoiding testing is the fear of recriminations from employers, insurance companies, and medical staff if a positive test becomes known to others. Yet immediate treatment for someone in the early stages of HIV disease is critical; therefore, early detection and reporting are important.
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December 14th, 2010 :: Filed under
HIV