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HIV Ab Window Period and Probability Theory

Apr 30, 2003

Dear Dr. Bob:

I hope you will review this and post if you agree with it.

Many readers of this forum seem to obsess about whether the HIV Ab test window period is 12 weeks or 90 days or 180 days based on generalized guidelines offered by the CDC and various Departments of Health in different States.

Here is my take on the issue.

The general guideline for the length of the window period before antibodies are detectable by the EIA test is based on data collected from a large number of people and applying statistics theory.

Without going into too much detail, the window period of 12 weeks is probably the time at which 99.9 or 98.6 or some very high percentage of people will sero-convert based upon statistics and probability theory. (I don't believe the exact percentage, which is ONLY an ESTIMATE in any case, is publicised).

Many readers are convinced that they are the statistical anomaly that fails to sero-convert and continue to worry even after a 24 week negative test.

If we apply probability theory, we can get a good idea of what the odds really are that a given person is the "statistical anomaly" after a negative HIVAb test result after 12 weeks.

Let's assume that there is only 1 chance in 10,000 that a person with a negative HIVAb test at 12 weeks after possible exposure is actually infected. Let's assume that the person engaged in one incidence of unprotected sex, which might on average transmit HIV in 1 of the cases. (Note: Playing Russian roulette even with 1 chamber loaded out of a hundred is still very dangereous).

The two events (probability of HIV transmission, and the probability of being the statistical anomaly in taking longer than 12 weeks to sero-convert) are independent.

So, the probability of being that statitical anomaly is approximately 1/100 * 1/10,000 = 1 in a million or less.

Yes, people do win the lottery every week, but in many cases a person who passes a 12 week antibody test has a better chance at winning the lottery than being HIV-infected, IMHO.

Having said that, if a person had a very high risk exposure: (blood transfusion with HIV infected blood, needle stick with HIV infected blood, unprotected sex with known HIV+ person etc) then it may make sense to get a second confirmation test at 24 weeks even after a 12 week negative test result.

But for all the people who are still worried about microscopic pores in condoms, oral sex, hand jobs etc after a 12 week negative antibody test, you should stop worrying unless you won the lottery already :-)

Response from Dr. Frascino


Sure, Ill post your statistical analysis and probability theory. I never really liked statistics in college, but when you boil it down to folks would have a much better chance of winning the lottery than being a late seroconverter, it makes all the numbers more palatable.

Thanks for the math lesson. Now, what are the real odds of finding a needle in a haystack? Stay well.

Dr. Bob

Condoms are not any gaurantee, are they?
Goodbye, farewell, and thank you

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