UNDERSTANDING TESTS FOR HIV: THE TESTS, TESTS FOR HIV AND TESTS FOR ANTIBODIES TO HIV
There are two general tests that detect HIV infection. Because HIV lives in blood cells, both are tests conducted on human blood, although tests using saliva and urine are expected soon. One blood test detects evidence of the virus itself; the other detects the antibody to the virus. Tests for HIV-Three tests for HIV infection detect either the virus itself or parts of the virus in the blood. These tests are called (1) cultures for the virus, (2) P24 antigen tests, and (3) polymerase chain reaction (PCR). Tests for HIV are more expensive, less well standardized, and less readily available than tests for the antibody to HIV. At present, the major uses of such tests are for the rare person whose test results for the antibodies to HIV are ambiguous, and for research studies. Tests for Antibodies to HIV-The most common method for detecting HIV infection is the test to detect antibodies to the virus. Antibodies are proteins the body makes to kill any microbe that invades human tissues. If antibodies are present, the microbe also is, or has been, present. Testing has been done to identify antibodies to many microbes for several decades; it is a common method for finding the microbes that cause a multitude of infectious diseases. Laboratories use two standard tests for detecting antibodies to HIV: an initial screening test called the ELISA, followed by a confirming test called the Western blot. The results of the tests are positive (meaning that the antibody is present), negative (meaning the antibody is not present), or indeterminant (meaning that the test results are inconclusive). Indeterminant, false negative, and false positive results. The antibody test, on rare occasions, produces indeterminant or false results. Indeterminant results mean that the laboratory cannot determine definitely whether the results are positive or negative (see below, “Indeterminant Test Results”). People with indeterminant results are usually told to repeat the test in three months. False results mean that the test results are inaccurate: they can be either falsely negative or falsely positive. A false negative result usually occurs because the test was taken too early during the course of the infection. After infection by most microbes, the body begins manufacturing antibodies within about two or three weeks. After infection with HIV, however, different people’s bodies produce antibodies over widely varying amounts of time: about half the people infected will produce antibodies and have positive blood tests within six weeks, most will have positive tests within three months, and some people do not produce antibodies after an even longer period, perhaps up to three years. During this early period in the infection—after infection but before antibodies are manufactured—tests can be falsely negative, meaning that the person actually has HIV infection but the antibody test is negative. The likelihood that a negative result is false is different for different people. A negative result is more likely to be false in people who are actively participating in high-risk behavior. A negative result is not likely to be false in people with low-risk behavior. For blood donors in general, the frequency of false negatives is vanishingly small: the standard antibody test will miss only 1 in 40,000 to 200,000 blood donors. The results of the tests can also be falsely positive. The results can be falsely positive because the laboratory made an error or mixed up blood samples, or because the person has antibodies to miscellaneous proteins that incidentally resemble HIV. If the laboratory is reliable, and if both the ELISA and Western blot are done, the frequency with which the tests are falsely positive is also vanishingly small. In a study done purposefully to magnify the number of false positive results, the frequency with which tests were falsely positive was 1 in 135,000 tests. The figures quoted above make the test for antibody to HIV one of the most accurate tests in medicine. Like other tests, it is subject to both human error and technological error. If there is reason seriously to question the results of the test, it is best simply to have it repeated. In the rare circumstance where repeat tests also leave questions, it is sometimes wise to take the test to detect the virus or parts of the virus.*253\191\2* totaldrugmart.com
