I have had two ELISA tests for HIV. The first time, the result was something like "rbt fresh sample" and the second time, the result was "initially reactive." I have had sex four times, but every time it was protected. The last time I had sex was about five years ago. I haven't had any symptoms. I read on the Internet that there are many reasons for the ELISA test to be false. I want to know the chances of not being infected by HIV and what I should do now.

R.

Let me begin by giving a little background on the tests for HIV (human immunodeficiency virus, the virus that causes AIDS). When people are infected with HIV, their bodies produce infection-fighting proteins called antibodies against the virus. We can diagnose HIV infection by testing the blood for the presence of these antibodies, using a variety of methods. The ELISA test (for enzyme-linked immunosorbent assay test) is the most commonly used antibody test. This test is very, very sensitive, which in scientific terms means that almost everyone who has HIV will test positive. The very few people with HIV who do not test positive have usually been tested in the brief "window period" between infection and the appearance of antibodies in the blood.

The ELISA test is also highly specific, which means that most patients who do not have HIV will test negative. However, some false-positive results do occur, so a second test is needed to confirm a positive ELISA. The Western Blot test is the most common confirmatory test for HIV. Many states in the United States do not allow labs to report the results of the ELISA test without first following up a positive result with a Western Blot. I never tell a patient that he or she has HIV until the result has been confirmed in this way. The Western Blot test looks for antibodies against several different parts of the virus. That makes the test very, very specific -- there are very few false-positive results. In fact, in a population with a low prevalence of HIV infection, the false-positive rate is about 0.0006 percent. That means out of a million people who get positive results on Western Blot tests, only six of them will turn out not to have HIV infection.

In your case, you had two ELISA tests that do not appear to have been confirmed with a Western Blot. I am not sure what "rbt fresh sample" on your first test means, but "initially reactive" on the second means the test was positive for HIV. If I were I you, I would get a Western Blot test. See my column on confidential HIV/AIDS testing for information on obtaining the test. If your Western Blot test is positive, you almost certainly have HIV. If it is negative, you should repeat the test in a few months. Some people will test positive with the ELISA test a few weeks before they test positive with the Western Blot.

People who have no risk factors for HIV and have a persistently positive ELISA but a persistently negative Western Blot usually do not actually have HIV. If you and your doctor still have doubts after multiple tests, you may want to have one of the newer PCR (polymerase chain reaction) tests that detect HIV itself rather than antibodies. These tests also occasionally have false-positive and false-negative results, but they can still be helpful.