What are the ELISA and Western blot tests?

The most common test for AIDS is the ELISA (enzyme-linked immunosorbent assay), which is performed on a blood sample. This test is very sensitive and detects almost all persons infected with the human immunodeficiency virus (HIV) except during the first few weeks of infection.

HIV breaks down the body's immune system, its natural defense system against foreign substances and invading organisms, such as bacteria that cause disease.

The ELISA test tests the body's immune response to HIV. The Western blot test determines the body's immune response to a specific HIV particle.

To be reported as positive by the ELISA test, at least 2 of 3 tests on the same blood specimen must react positively. In a small number of cases, people may have a false-positive test; the test may show presence of infection but the test result may be caused by rheumatoid arthritis or an unidentified factor, not HIV infection.

Thus, positive ELISA specimens are confirmed with another more specific test, usually the Western blot test. The Western blot test is less sensitive than the ELISA test but it hardly ever gives a false-positive result.

Of every 10 million people in the U.S. tested for HIV infection, approximately 25,000 (0.25%) repeatedly test positive by ELISA, but only 10,000 (0.1%) are confirmed by Western blot. In high-risk groups where HIV infection rates are as high as 30% to 70%, a positive ELISA test is almost always confirmed by Western blot.

Anyone with a positive test for HIV infection should seek follow-up medical evaluation, interpretation of the results, counseling, and advice for possible lifestyle changes. A negative HIV antibody test implies that you probably have not been infected with the HIV virus.

If the results of the same test done 6 months later are still negative for HIV antibodies, it is highly unlikely that you have been infected with the virus or that you will develop AIDS. This assumes that in those 6 months you have not engaged in any high-risk activities.

Your doctor will use the findings of these tests to determine your degree of risk and whether you have been infected with HIV. He or she will discuss with you how to proceed, what precautions to take, and what follow-up steps are important.