What Kinds Of Blood Tests Are Used?

The blood tests most commonly used to diagnose HIV infection work by measuring the levels of antibodies produced by the body against HIV. Antibody-detecting assays, or tests, include the:

• Enzyme immunoassay (EIA)
• Enzyme-linked immunosorbent assay (ELISA)
• Western blot test

Usually, the first test that laboratories use to detect the presence of HIV antibodies is an EIA or the ELISA. If the first test produces a positive result (HIV antibodies appear to be present), then the more sensitive Western Blot test is used to confirm it.

EIA or ELISA tests take from one to two weeks to complete, depending on where the test is performed.

Why Is It Often Necessary To Repeat An HIV Test?

Although a negative result on an HIV blood test usually means that the person is not infected with the virus, that is not always the case. The body may take three to six months after exposure to the virus to produce enough antibodies to be detectable in the bloodstream.

Because of this delay between infection and the appearance of HIV antibodies, a person should be retested six months after the last possible exposure to HIV. It is also important to remember that a person who has been exposed to HIV can pass the virus to others even before HIV antibodies appear in the bloodstream.