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negative p-24 and antibody test The Body: Rick Sowadsky M.S.P.H., C.D.S, Answers to Safe Sex Questions
Jan 16, 1997

Dear Mr. Sowadsky Thank you very much for your time - I had a low risk exposure with someone whose HIV status is unknown to me (oral sex where the man did not ejaculate in my mouth). Still, I was concerned and I had a p24 test one month after the incident, and then I had an antibody test after 2 months; both negative. I am planning on having another test at 6 months, but can I at least be somewhat reassured by these results? My Dr. tells me that it would be highly unusual for someone to later test positive after such results, and says that I don't need to be tested again.

Response from Mr. Sowadsky

Hi. Thank you for your question.

The following will go into detail when the p-24 antigen test and the antibody tests show positive in an infected person:

The p-24 antigen test will pick up an infection an average of 6 days prior to a person testing positive on the antibody tests. The p-24 antigen tests will often show positive 2-3 weeks after initial infection. However, not all recently infected persons have detectable levels of p-24 antigen in their blood. Therefore having a negative p-24 antigen test after a recent exposure is not considered a conclusive or definitive test result.

A negative p-24 antigen test does not necessarily mean the person is not infected. Once the person turns antibody positive, the p-24 antigen test will often revert back to a negative result. So this test will only be positive for a short period of time.

The antibody tests show positive an average of 25 days after infection.

The usual period of time that most (but not all) people will show positive on the antibody test is 3 months. The maximal amount of time that a person starts to show antibody positive (in more than 99% of cases) is 6 months.

So we can say that the antigen tests will pick up an infection about 6 days before the antibody tests will turn positive. Getting an antigen test at 1 month will not always pick up the infection. Let me also stress that the p-24 antigen test alone CANNOT be used to diagnose HIV infection. Remember, this test will not always pick up an infection. A p-24 antigen test (whether positive or negative) must always be used alongside other HIV diagnostic tests, before a persons HIV status can be determined.

If you tested negative on the p-24 antigen test at 1 month, and negative on the antibody test at 2 months, these are still not conclusive tests for HIV infection. Only a negative antibody test at 6 months is considered conclusive on these tests. But certainly, having the test results negative so far are encouraging, especially if your risk was low, and he did not ejaculate in your mouth. Infection through pre-cum is still possible, but not nearly as likely as if he had ejaculated in your mouth. So your test results are encouraging but not definitive at this point.

If you have any further questions, please feel free to contact the Centers for Disease Control at 1.800.232.4636 (Nationwide).



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