Usually, HIV infection is detected by an HIV antibody test. The first test to be done, usually on blood, but possibly on saliva, is an enzyme linked immunosorbent assay (ELISA). Since this test can sometimes be positive even when someone is not infected a `false positive' a second test called the Western blot is done.

The amount of time between getting HIV infection and developing antibodies varies very widely. The vast majority of people with HIV will produce antibodies by around 45 days after infection. However, in a small proportion it may take up to six months for antibodies to develop, and in a very few people with HIV infection it may take even longer. This is one reason why a lack of HIV antibodies does not always mean freedom from infection.

There are also a number of tests which can look for the virus or parts of the virus itself (antigen testing and RNA viral load testing), damage to the immune system, or other aspects of the body's response to the effects of the virus. These should not be confused with the HIV antibody test.