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Window period
Jul 8, 2005

Dear Dr. Bob: Best regards from Belgium! You do a fantastic job that is highly appreciated worldwide (no exaggeration). I would like to ask you a question. As a statistics shows the probability of getting HIV from a HIV+ in an insertive vaginal intercourse is about 5 in 10,000. If a receptive person is of unknown status you have to multiply the probability by 1/200. Thus, if a person having sex with an unknown HIV status partners has odds to be infected only 2,5 in 1,000,000.

Suppose you as an insertive person and is tested negative after 2 weeks. Your probability to be infected becomes 1 in 1 Million! If you are tested negative in 3 weeks after exposure, the probability is: 4 in 10 Million, in 4 weeks 4 in 100 Million or 1 in 25,000,000. In practical terms, the probability is zero.

If your partner was definitely HIV positive, your chances are somewhat less: if you are tested negative in 4 weeks your odds of getting HIV are 1 in 100,000 and you probably need another test to be 100% sure.

Let us see what would happen if you are tested negative in 6 weeks after an exposure with an HIV+ person. Your odds of getting HIV from an HIV+ person in an insertive sex in this case are 3 out of 1,000,000 while in 8 weeks is 7 in 100,000,000 or less than 1 chance in 10,000,000 which is practically zero.

If a person is receptive in vaginal intercourse, you should multiply these numbers up to 20! Thus, if you are tested negative in 8 weeks after receptive intercourse your chances of getting HIV from a definitely HIV+ partner are 2 in 1,000,000, which is also conclusive, but if you tested negative after 4 weeks, the chances are 1 in 5,000.

The question is: why people go through torture waiting more than 2 months if after 2 months the test for receptive and insertive vaginal sex is absolutely conclusive?

This also explains why Tony (in your Forum) is surprised having sex with 3000 persons of unknown status and not getting HIV: having sex with 3000 women of unknown status, Tony had 6 chances out of 1000 to be infected or only 1 in 166! All odds were in favour of Tony, though he played Russian roulette aiming a gun barrel at his temple.

Response from Dr. Frascino

Hi Belgium-Guy,

Let me guess, you're some kind of frustrated math teacher, aren't you???

Actually estimated statistical risks cannot be combined in the way you suggest.

Why do we suggest folks wait for three months before regarding their HIV-antibody tests are definitive? The answer is complex and has to do with limitations of the test (sensitivity and specificity of identifying detectable levels of anti-HIV antibodies) and variabilities in individual immune responses (integrity of individual immune response and time necessary to make detectable levels of anti-HIV antibodies). Yes, the vast majority of folks who become HIV positive will test positive on HIV-antibody tests at two months. But a "vast majority" really isn't good enough when one considers the consequences of missing an HIV diagnosis, right?

As for Tony, if he's even for real, that boy has issues!

Stay well, Belgium-Guy.

Dr. Bob


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