What is the false-positive rate for an HIV PCR test and when, after exposure, can it be taken?

Try this site, for a fairly good FAQ: http://www.avert.org/hivtesting.htm

The Wikipedia article on HIV testing is really interesting. I would especially read what it has to say about false positives (ie, has s/he been recently ill? Does s/he have allergies?)

To quote: “Other studies have confirmed the accuracy of current methods of HIV testing in the United States, reporting false-positive rates of 0.0004% to 0.0007% and false-negative rates of 0.003% in the general population.”

I think it is fairly telling about the P24 that it is no longer used in the US or EU.

Edited to add: Having said that, I personally know someone who got a false positive. If it happens, you can never donate blood again. That’s how he got his—after a blood drive, Red Cross wrote him to say basically “you don’t have HIV, but you did get a false positive on the first test we did so we’re never taking your blood again”.

@pdworkin I’ve been on that site this morning – doesn’t say much, though it is simple to read and look at.

I understand your friend is concerned, but a lot depends on the type of exposure as to how concerned he should be, and whether he should have started antiretrovirals (which can be offered in high risk cases). The nucleic acid tests are approved for diagnosis of acute HIV, but as you said, depending on the test they can have a high false positive rate. In any event, all initial positive tests are followed up with a second confirmatory test. My suggestion to your friend (again, I don’t know the severity of the exposure) is to wait for a total of 3 weeks (i.e. one more week since he’s already waited 2) and then get the ELISA. If that is negative, he can test again in 2 months. If that is negative, it is unlikely he contracted HIV. All of this should be discussed with his primary medical doctor. This website has useful information in lay terms about testing, and this one about HIV itself.

He should be reassured that HIV has a remarkably low transmission rate, even with high risk inoculations, so his odds of contracting HIV remain low.

@shilolo we were under the impression that antiretrovirals are given in rape cases or if one goes into the emergency room post some sort of trauma – this was a voluntary encounter where the condom ripped – we, since I wrote up this q, set up a PCR test for him for this Friday, I’ll be going over there where he is to be there for the results the Thursday, about one week after – he’s been talking to many counselors and I’m 24/7 on call for him…he did talk to his doctor, doctor gave him a Zanax prescription which is something I do not think he should get into

@Simone_De_Beauvoir Not exactly. Rape does not have to be involved in the least. Post-exposure prophylaxis can be offered for individuals with mucous membrane contact with blood or secretions from someone with high risk features (and of course, known HIV). At this point, the discussion is academic since it has been >2 weeks since the event, but something worth considering for future events.

@shilolo no clue if that person is positive – it was an anonymous encounter