Two types of tests are commonly used to detect HIV infection. The most commonly used initial test is an enzyme immune assay (EIA) or the enzyme-linked immunosorbent assay (ELISA).

If EIA test results show a reaction, the test is repeated on the same blood sample. If the sample is repeatedly reactive, the results are "confirmed" using a second test such as the Western blot. This more specific (and more expensive) test can tell the difference between HIV antibodies and other antibodies that can react to the EIA and cause false positive results.

False positive EIA results are uncommon, but can occur. A person is considered infected following a repeatedly reactive result from the EIA, confirmed by the Western blot test.

In addition to the EIA or ELISA and Western blot, other tests now available include:

• Radio-immunoprecipitation assay (RIPA): A confirmatory blood test that may be used when antibody levels are very low or difficult to detect, or when Western blot test results are uncertain. An expensive test, the RIPA requires time and expertise to perform.

• Dot-blot immunobinding assay: A rapid-screening blood test that is cost-effective and that may become an alternative to standard EIA and Western blot testing.

• Polymerase chain reaction (PCR): A specialized blood test that looks for HIV genetic information. Although expensive and labor-intensive, the test can detect the virus even in someone only recently infected.