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P24 antigen test: Good when no antibodies found?

Sep 12, 1996

Hi. I recently tested negative for 1 yr after possible exposure. I believe that only the standard Elisa test was performed. While no antibody was detected and tests are supposed to be > 99.99% accurate, I have read of some people who develop antibodies much later than the window period. I also recently heard of a p24 antigen test which is supposed to be very sensitive when there are no antibodies present (ie. lab settings). Can I request a p24 antigen test, and if I come out negative, can I then conclude that I'm OK (given the fact that I've practiced safe sex since that incident last year?) Thanks for answering my question

Response from Mr. Sowadsky

Hi. Thank you for your question.

As you have stated yourself, the antibody tests are extremely accurate (as long as it's been more than 6 months after a possible exposure to the virus). That's as good as any test in medicine could ever be. So if you have not put yourself at risk 6 months prior to the test, that would indicate (with more than 99% accuracy) that you are not infected with HIV. Assuming you have not put yourself at risk after taking the test, there would be no reason to believe you currently have HIV, and therefore there would be no reason to repeat the test.

It is very unusual for persons to take longer than 6 months to test positive on antibody tests, but that does happen from time to time. There are cases in the medical literature of people taking longer than 6 months, but remember that those cases are very rare.

As to the p-24 antigen tests, these tests are not designed for HIV diagnosis by themselves. They must be used alongside antibody tests. Their only approved use (in the United States) is as a supplementary screening test in the American blood supply. In the American blood supply, both antibody tests and p-24 antigen tests are used for screening. It's important to remember that a p-24 antigen test by itself cannot be used to diagnose HIV. On average, in an infected person, the p-24 antigen test will show positive about 1 week before an antibody test will pick up the infection. When an antibody test turns positive, the p-24 antigen test will often revert back to negative. So the p-24 antigen test will only shorten the window period for testing by about 1 week.

In summary, since you waited longer than 6 months to be tested (1 year in your case), your negative test is considered extremely accurate, and there is no need for p-24 antigen testing. Also since you have not put yourself at risk of infection since taking that test, there is no reason to believe that you're currently infected with HIV.

If you have further questions, please e-mail me at "" or call me at 1-800-842-AIDS.

Conflicting HIV test info
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