HIV infection is commonly diagnosed by blood tests. There are three main types of tests that are commonly used: (1) antibody tests, (2) RNA tests, and (3) a combination test that detects both antibodies and a piece of the virus called the p24 protein. In addition, a blood test known as a Western blot is used to confirm the diagnosis.
No test is perfect. Tests may be falsely positive or falsely negative. For example, it can take some time for the immune system to produce enough antibodies for the antibody test to turn positive. This time period is commonly referred to as the "window period" and may last six weeks to three months following infection. Therefore, if the initial antibody test is negative, a repeat test should be performed three months later. Early testing is crucial, because early treatment for HIV helps people avoid or minimize complications. Furthermore, high-risk behaviors can be avoided, thus preventing the spread of the virus to others.
Testing for HIV is usually a two-step process. First, an inexpensive screening test is done. If that test is positive, a second test (Western blot) is done to confirm the result. Antibody tests are the most common initial screening test used. There are different types of antibody screening tests available:
Most commonly, blood is drawn for an enzyme immunoassay (EIA). The test is usually run in a local laboratory, so results can take one to three days to come back.
Other tests can detect antibodies in body fluids other than blood such as saliva, urine, and vaginal secretions. Some of these are designed to be rapid tests that produce results in approximately 20 minutes. These tests have accuracy rates similar to traditional blood tests.
HIV home-testing kits are available at many local drug stores. Blood is obtained by a finger prick and blotted on a filter strip. Other test kits use saliva or urine. The filter strip is mailed in a protective envelope to a laboratory to be tested. Results are returned by mail in one to two weeks.
All positive antibody screening tests must be confirmed with a follow-up blood test called the Western blot to make a positive diagnosis. If the antibody test and the Western blot are both positive, the likelihood of a person being HIV infected is >99%. Sometimes, the Western blot is "indeterminate," meaning that it is neither positive nor negative. In these cases, the tests are usually repeated at a later date. In addition, an RNA test for the virus might be done.
Other tests, such as those that look for virus RNA and the combination test, are not commonly used for screening.
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