The ELISA is used as a screening test. A positive result does not necessarily mean that the subject has HIV infection as there are certain conditions that may lead to a false positive result (such as Lyme disease, syphilis, and lupus). A positive ELISA test is always followed by a confirmatory test termed Western Blot. A positive western blot is generally regarded as conclusive for an HIV infection.

Negative tests do not necessarily rule out HIV infection, because there is an interval (called the "window period") between HIV infection and the appearance of measurable anti-HIV antibodies. So, if someone is suspected of having acute or primary HIV infection and being in the "window period," a negative HIV ELISA and Western Blot will not rule out HIV infection. Additional testing with HIV viral load or p24 antigen testing will need to be performed.