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HIV testing and the P24 test

Jul 13, 2008


Thanks for answering my question, I'm finding it really difficult to find the correct information as there's so much conflicting evidence out there.

5 weeks ago I received unprotected oral sex from someone I don't know in a gay sauna. I was pretty drunk at the time, so don't remember everything that happened, but am pretty sure that was the only risky sexual activity which happened.

3 weeks later I went to the sexual health clinic and had swabs and TPHA, all of which came back negative.

But since then for the past 2 weeks I have had diarrhoea every day, no other symptoms of seroconversion.

I had a HIV antibody test and P24 5 weeks after the event which came back negative.

My question is after the negative P24 test do I need to have a repeat HIV test at 12 weeks? Or is the negative P24 reassuring enough as this private company recommends:

Thanks for your advice and support.

Response from Dr. Horwath

The HIV antibody screening tests are the most reliable tools to diagnose HIV infection.

The p24 antigen test detects the presence of a protein in HIV. However, the p24 test relies on a certain level of virus in the bloodstream, and a person may be infected but may not actually have a positive p24 test. One study demonstrated that the p24 test failed at detecting infection in 75% of infected blood donors. It is not a very sensitive test.

On the other hand the p24 test is very specific. A positive result on a p24 antigen test is 99.9% accurate; false positives are highly unlikely. This is also true of the Western blot, which is the confirmatory test used in current HIV antibody tests.

The bottom line is that the HIV antibody test (ELISA and Western blot) is still the most reliable test available. If you have a negative HIV antibody test 3 months after an exposure, then you should be reassured that you do not have HIV infection.

My advice to you is don't get drunk before you engage in sex. Intoxication with alcohol or drugs impairs your judgment and leads to making poor decisions about sex and self-protection.

My husband is HIV POSITIVE border line Aids

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