How do HIV tests work?
What are the different HIV screening tests available in the United States?

How do HIV tests work?

Once HIV enters the body, the immune system starts to produce antibodies – (chemicals that are part of the immune system that recognize invaders like bacteria and viruses and mobilize the body's attempt to fight infection). In the case of HIV, these antibodies cannot fight off the infection, but their presence is used to tell whether a person has HIV in his or her body. In other words, most HIV tests look for the HIV antibodies rather than looking for HIV itself. There are tests that look for HIV's genetic material directly, but these are not in widespread use.

The most common HIV tests use blood to detect HIV infection. Tests using saliva or urine are also available. Some tests take a few days for results, but rapid HIV tests can give results in about 20 minutes. All positive HIV tests must be followed up by another test to confirm the positive result. Results of this confirmatory test can take a few days to a few weeks.

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What are the different HIV screening tests available in the United States?

In most cases the EIA (enzyme immunoassay), used on blood drawn from a vein, is the most common screening test 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 to make a positive diagnosis. There are EIA tests that use other body fluids to look for antibodies to HIV. These include

• Oral Fluid Tests – use oral fluid (not saliva) that is collected from the mouth using a special collection device. This is an EIA antibody test similar to the standard blood EIA test. A follow-up confirmatory Western blot uses the same oral fluid sample.
• Urine Tests – use urine instead of blood. The sensitivity and specificity (accuracy) are somewhat less than that of the blood and oral fluid tests. This is also an EIA antibody test similar to blood EIA tests and requires a follow-up confirmatory Western blot using the same urine sample.

Rapid Tests:

A rapid test is a screening test that produces very quick results, in approximately 20 minutes. Rapid tests use blood from a vein or from a finger stick, or oral fluid, to look for the presence of antibodies to HIV. As is true for all screening tests, a reactive rapid HIV test result must 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. Please visit the rapid HIV testing section for details.

Home Testing Kits:

Consumer-controlled test kits (popularly known as "home testing kits") were first licensed in 1997. Although home HIV tests are sometimes advertised through the Internet, currently only the Home Access HIV-1 Test System is approved by the Food and Drug Administration. (The accuracy of other home test kits cannot be verified.) The Home Access HIV-1 Test System can be found at most local drug stores. It is not a true home test, but a home collection kit. The testing procedure 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 are given an identification number to use when phoning in for the results. Callers may speak to a counselor before taking the test, while waiting for the test result, and when the results are given. All individuals receiving a positive test result are provided referrals for a follow-up confirmatory test, as well as information and resources on treatment and support services.

RNA Tests

RNA tests look for genetic material of the virus and can be used in screening the blood supply and for detection of rare very early infection cases when antibody tests are unable to detect antibodies to HIV.

For a list of HIV tests that are FDA-approved, visit the Food and Drug Administration (FDA) Center for Biologics Evaluation and Research.

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