A positive result on the ELISA screening test does not mean that the person has HIV infection. Certain conditions may lead to a false positive result, such as Lyme disease, syphilis, and lupus.
A positive ELISA test is always followed by a Western blot test. A positive Western blot confirms an HIV infection. A negative Western blot test means the ELISA test was a false positive test. The Western blot test can also be unclear, in which case more testing is done.
Negative tests do not rule out HIV infection. There is a period of time, called the window period, between HIV infection and the appearance of anti-HIV antibodies. During this period, antibodies usually cannot be measured.
If a person might have acute or primary HIV infection and is in the window period, a negative HIV ELISA and Western blot will not rule out HIV infection. More tests for HIV are needed.