There are three types of tests used to diagnose HIV infection: antibody tests, antigen/antibody tests, and nucleic acid tests (NATs). HIV antibodies are disease-fighting proteins that the body produces in response to HIV infection. NATs look for the virus in blood. Your health care provider can determine the appropriate HIV test for you.  How soon each test can detect HIV infection differs, because each test has a different window period. The window period is the time between when a person may have been exposed to HIV and when a test can accurately detect HIV infection.

• Antibody tests check for HIV antibodies in blood or oral fluid. Most rapid tests and home use self-tests are antibody tests. Generally, antibody tests that use blood from a vein can detect HIV sooner than tests done with blood from a finger stick or with oral fluid.
• Antigen/antibody tests can detect both HIV antibodies and HIV antigens (a protein of the virus) in the blood and are the most used HIV tests. After HIV exposure, antigens will show up in the blood of a recently exposed person sooner than antibodies.
• NATs look for HIV in the blood taken from a vein. These tests may also be called "viral load tests" because they not only detect the virus but also determine the quantity of virus present in the blood. They can usually find HIV as soon as 10 to 33 days after infection. They are mostly used for monitoring HIV treatment and not for routine screening because they are expensive.

A person’s initial HIV test will usually be either an antibody test or an antigen/antibody test. NATs are not routinely used for HIV screening unless the person had a high-risk exposure or a possible exposure with early symptoms of HIV infection.

When an HIV test is positive, a follow-up test will be conducted for confirmation.  Sometimes people will need to visit a health care provider to take a follow-up test using another blood sample. Other times, the follow-up test may be performed in a lab using the original blood sample to ensure that the first positive result wasn’t a false positive.

Talk to your health care provider who will determine your HIV risk factors and the best type of HIV test for you.