Frequently Asked Questions About HIV/AIDS (Part Two)
In regard to the accuracy of the antibody tests:
For the absolute most accurate test result, you must wait six months after your last possible exposure to the virus (or anytime after that). At six months, the tests are more than 99% accurate. If you get tested before the six-month waiting period, it's possible (though rare) that you could have the infection but the test won't pick it up.
When a person gets an antibody test for HIV, the first test that is done is the ELISA test, also called the EIA. If the ELISA test is negative, this is considered a negative result, and confirmatory tests are not necessary. A negative result means that no antibodies were found.
If the ELISA test is repeatedly positive, it must be confirmed with a confirmatory test (usually a Western Blot or an IFA test). This second test can be done from the same tube of blood. You must test positive on both the ELISA test and the confirmatory test, to be considered HIV positive. If both the ELISA test and the confirmatory test are positive, this is considered a positive result. This means that antibodies were found, and the person is infected with HIV.
If the ELISA test is repeatedly positive but the confirmatory test is negative, this is considered a negative result overall, and this means that no antibodies were found.
ELISA tests are either positive or negative. Western Blot tests however, can be positive, negative or indeterminate. An indeterminate test means that the test could not determine whether a person is infected or not. This can be due to two things. Either a person was so recently infected that they are just starting to produce antibodies, or something else is cross-reacting on the test. If an indeterminate test result occurs, the person is usually re-tested in four to six weeks to determine whether they are infected or not.
The ELISA test is very good at picking up infections, but sometimes gives false positive readings (which is why confirmatory testing is always done on positive ELISA results). The Western Blot is very good at ruling out false-positive ELISA tests.
This article was provided by Rick Sowadsky, M.S.P.H..