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test question
May 28, 2001

How reliable is the p24 test? If one tests positive for this do they usually become positive for the antibodies?

Response from Mr. Kull

Let me preface this response by repeating my mantra: the HIV antibody test is the most reliable tool to diagnose HIV infection in a person. There is not another screening test that, when used alone, beats the sensitivity and specificity of the EIA and Western Blot screening sequence.

The p24 antigen test detects the presence of a protein on the surface of HIV (the "p" standing for protein, and "24" standing for its molecular weight). P24 is referred to as an antigen because it provokes the body's immune response, causing it to generate antibodies. Since HIV is present in the body in very high levels before antibodies are produced, the p24 test may be useful in detecting infection when antibody tests cannot. For this reason, p24 tests are useful for screening the blood supply and for diagnosing acute infection. P24 antigen tests probably detect infection only a week earlier than the antibody tests. A positive result on a p24 antigen test is 99.9% accurate; false positives are highly unlikely.

Here's the catch: since the p24 test relies on a certain level of virus in the bloodstream, it is possible that someone may be infected but may not actually have a positive p24 test. Also, when a person's immune system produces antibodies, the p24 antigen test has a much more difficult time detecting infection (due to something called antibody-antigen complexing). Antibody levels, once established, remain persistent throughout infection. One study demonstrated that the p24 test failed at detecting infection in 75% of infected blood donors. It is not a very sensitive test.

So, a positive p24 antigen result is very reliable (this should always be confirmed at some point with an antibody test and an accurate risk history). A negative p24 antigen test does not say a whole lot.

RMK



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