How do HIV tests work?

The most commonly used HIV tests detect HIV antibodies – the substances the body creates in response to becoming infected with HIV. There are tests that look for HIV's genetic material or proteins directly; these may also be used to find out if someone has been infected with HIV.

It can take some time for the immune system to produce enough antibodies for the antibody test to detect, and this “window period” between infection with HIV and the ability to detect it with antibody tests can vary from person to person. During this time, HIV viral load and the likelihood of transmitting the virus to sex or needle-sharing partners may be very high. Most people will develop detectable antibodies that can be detected by the most commonly used tests in the United States within 2 to 8 weeks (the average is 25 days) of their infection. Ninety-seven percent (97%) of persons will develop detectable antibodies in the first 3 months. Even so, there is a small chance that some individuals will take longer to develop detectable antibodies. Therefore, a person should consider a follow-up test more than three months after their last potential exposure to HIV. In extremely rare cases, it can take up to 6 months to develop antibodies to HIV.

Conventional HIV tests are sent to a laboratory for testing, and it can take a week or two before the test results are available. There are also rapid HIV tests available that can give results in as little as 20 minutes. A positive HIV test result means that a person may have been infected with HIV. All positive HIV test results, regardless of whether they are from rapid or conventional tests, must be verified by a second “confirmatory” HIV test.