General: As soon as the virus enters the body, the immune system produces antibodies, which are chemicals that locate invaders and fight off infections. While these antibodies cannot successfully destroy the virus, their presence can be used to detect whether HIV is in the body.
It can take some time for the immune system to produce enough antibodies for the antibody test to detect them. This time period, known as the "window period," varies greatly among patients. Most people will develop detectable antibodies within two to eight weeks (the average is 25 days). However, some individuals might take longer to develop detectable antibodies. Ninety-seven percent of people develop antibodies in the first three months following the time of their infection. In very rare cases, it can take up to six months to develop antibodies to HIV. Therefore, if the initial negative HIV test was conducted within the first three months after possible exposure, repeat testing should be considered at a time longer than three months after the exposure.
Enzyme immunoassay (EIA): The most common HIV tests use blood to detect HIV infection. In most cases, the enzyme immunoassay (EIA) is used to look for antibodies to HIV. A positive (reactive) EIA must be used with a follow-up (confirmatory) test, such as the Western blot test, to make a positive diagnosis. A positive diagnosis means that a person is infected with HIV.
Western blot test: A Western blot test is typically used to confirm a positive HIV diagnosis. During the test, a small sample of blood is taken and used to detect HIV antibodies (not the HIV virus).
Saliva or urine: Tests using saliva or urine are also available. This saliva or urine test is similar to the standard blood EIA test. However, they tend to be less sensitive and accurate than blood and oral fluid tests. A follow-up confirmatory Western Blot uses the same sample.
Oral fluid test: An oral fluid test involves oral fluid (not saliva) that is collected from the patient's gums. This oral fluid test is similar to the standard blood EIA test. A follow-up confirmatory Western Blot uses the same oral fluid sample.
RNA tests look for genetic material of HIV. These tests can be used to screen the blood supply and to detect very early infection in those rare cases when antibody tests are unable to detect antibodies to HIV.
Rapid test: A rapid test produces results in about 20 minutes. Rapid tests use blood from a vein or from a finger stick or oral fluid to look for HIV antibodies. A positive HIV test should be confirmed with a follow-up confirmatory test before a final diagnosis of infection can be made. These tests have similar accuracy rates as traditional EIA screening tests.
Home testing kit:
Consumer-controlled test kits (popularly known as "home testing kits") were first licensed in 1997. The Home Access HIV-1 Test System® is the only home kit that is approved by the U.S. Food and Drug Administration (FDA). The Home Access HIV-1 Test System® is sold at most local drug stores. It is not a true home test, but rather a home collection kit. The test involves pricking a finger with a special device, placing drops of blood on a specially treated card, and then mailing the card in to be tested at a licensed laboratory. Customers receive an identification number to use when calling for the results. Callers may speak to a counselor before taking the test, while waiting for the test result, and/or after the results are given. All individuals who receive a positive test result are given referrals for a follow-up confirmatory test, as well as information and resources on treatment and support services.