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Viral load at 3 months ?
Apr 17, 2001

Dear sir, What test would you recommend at 3 months post high risk exposure: a RNA PCR( viral load ), or a DNA PCR. Which one of this is most accurate? What would you say about the acuracy of Ag P24 & ELISA Antibody both -ve after the 3 months exposure ( giving the fact that I've been ARS like symptomatic for almost 2 months now.) Would you think that a negative PCR test at 3 months is final ? Thank you very much, I'm looking forward to your answer.

Response from Dr. Little

Both a DNA and an RNA PCR may be quite sensitive for the detection of virus 3 months following an exposure. Niether test is approved for use in the diagnosis of HIV infection and should always be confirmed by more traditional HIV antibody testing. Also, it is worth inquiring what material is being used for the DNA PCR test. If plasma is used for this test, then I am not as confident that the test will reliably detect virus as early as 3 months post exposure. If blood cells (peripheral blood mononuclear cells - PBMC) are used for the DNA PCR test, then I would expect the sensitivity of the test to be better.

Regarding a p24 antigen, this also can be a very good test, but it's major advantage is cost - it is probably not as sensitive as a plasma HIV RNA PCR.

If your symptoms represent an acute HIV viral illness, then in general a plasma HIV RNA PCR is the most sensitive available test, though again, I must emphasize, not approved for use in the diagnosis of HIV infection. While I think that a negative RNA PCR test would make HIV infection much less likely at 3 months post-expousre, there is not enough data in this setting to say that it is definitive. I would have follow-up antibody testing at 6 months to be more certain, especially since you had symptoms which sounded like a viral syndrome. Appropriate counselling regarding your exposure risk and the need for follow-up testing may also help to sort this out.

The setting where I have seen a negative HIV RNA PCR was in someone who had a recent exposure and a viral illness for which they sought medical attention. This individual turned out to have chronic HIV infection (probably of several years duration) and a viral (ie. non-HIV viral) syndrome. The reason his viral load was undetectable, is that some people do have better than average immune responses to infection and in this case his was sufficient to keep his viral load less than 500 copies. This is unusual, but again, one of the limitations of using HIV RNA as a diagnositic tool.

What causes resistance?
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