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Probability and Conclusiveness of HIV test

Apr 22, 2013

Hello Dr. Lisa Hightwo-Weidman,

I went through a period of time where I had a lot of sexual partners. I'm a gay male and I typically bottom, so I guess that puts me in the "high risk" category. I had a really scary incident where I took an HIV antibody test and read the results incorrectly (I'm in a foreign country, the results in a different language) and thought I was positive for about 24 hours. What a relief it was when I found out I wasn't!

I started researching a lot about HIV and from what I've gathered, HIV tests are never 100% accurate but are greater than 99% accurate at about 13 weeks. I'm curious about the conclusiveness of HIV tests and how health professionals think about them in terms of probability. Here is my logic: If you turn the 13 week >99% percentage into a fraction denoting probability and multiply that with the probability of becoming infected with a single sexual act, the probability of being infected, depending on the act, may be 1 in >100,000. That sounds pretty conclusive!

But let's take somebody who is at high risk like me. Let's say I had receptive anal sex with a stranger I met from the internet of an unknown status, he doesn't wear a condom and ejaculates inside of me. If he was HIV positive, that gives me (from what I've found) a 1 in 50 chance of contracting HIV. I got an 8 week antibody test with a negative result, and (from information I've found) 95% of people seroconvert before 8 weeks. That's 1 in 20 that don't, but will eventually seroconvert. So in this situation, I have a 1 in 1000 chance of seroconverting and becoming HIV positive by 13 weeks. That sounds okay, but not awesome.

I'm wondering what probability, if any, is accepted within the HIV specialist community as "conclusive?" It seems that with many straight people engaging in vaginal sex, an 8 week test would be considered conclusive by some doctors. But with gay people engaging in anal sex, 8 weeks isn't good enough. At what point can we say "it's conclusive?" Why are there differences in national standards and places like Massachusetts that have specifically conducted studies on MSM and accept 6 weeks as a conclusive antibody test? Some insight would be great!

By the way, I was tested 8 weeks after having protected receptive sex with someone of unknown status, so I'm feeling *mostly* confident about receiving a negative result after 13 weeks.

Thanks for being such a great resource!


Response from Dr. Hightow-Weidman

Hello there The sensitivity of HIV testing has improved greatly over the past few years. Current antibody tests can pick up HIV infection earlier than prior tests and by 90 days practically all (97%) infections are detected by antibody testing alone the remainder turn positive by 180 days. Therefore, a negative HIV antibody test after 90 days means that infection was highly unlikely and a test that is negative at 180 days or beyond is considered conclusive for the absence of HIV infection. Even newer HIV tests look for the virus itself. These include assays such as HIV RNA tests (viral load) and p24 antigen tests. These become detectable within days of infection. So, an undetectable HIV RNA or p24 antigen test accompanied by a negative HIV antibody test at 90 days or beyond can also be considered conclusive for the absence of HIV infection. Hope this helps. Take care, LHW

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