[Question #5314] Planned Parenthood warning about Duo HIV test and numbers, interesting!

22 months ago
Hello Doctors:

I have a question that I believe most readers here with anxiety will appreciate.

Yesterday, 27 days post me performing analingus on a CSW of unknown status, I went to a Planned Parenthood to get a 4th gen ag/ab Alere blood test done.  I was warned by the technician/lab person that 1 in 2000 tests generate a false positive result.  My result was thankfully negative.

I find this interesting and concerning in the context of texting.

Dr. Hansfield recently fielded a question very similar to my own, and assessed the following:

Let's go through some numbers. If your partner was at average risk for HIV, the chance she had HIV probably was no higher than 1 chance in a thousand she had HIV. If a sex worker, maybe up to 1% chance, no higher. Chance of transmission:  let's use CDC's figure of 1 chance in 20,000 for mouth to penile transmission. (There have been no cases known by cunnilingus, so we can ignore it. Or if you like, use the same figure as for fellatio). Next, let's say your current test results are 99% reliable, i.e. 1% chance you are infected and the test missed it. With these figures, the odds you have HIV can be calculated as 0.001 x 0.00005 x 0.01 = 0.0000000005. If I've correctly counted the decimal places, that's 1 chance in 2 billion. In other words, zero.

Yesterday, he claimed my situation, which had considerably less exposure than the mutual unprotected oral, was 1 chance in millions post a 27 day negative duo test result:

' Combining that with the near zero risk of the exposure, and it calculateds to less than one chance in millions you have HIV, i.e. zero for all practical purposes.'

My first question is was this a type-0?  Did he mean 1 in BILLIONS?  I think he must have, correct?

My second question, which i think many posters here will appreciate, is the nature of risks in testing.  If your chances of HIV infection are 1 in millions (or billions for me, now, right?) isn't the 1 in 2000 chance of receiving a false positive result more risky and concerning to overall anxiety relief?

I am genuinely curious as to your feelings on this issue.  Is testing NOT recommended under certain extreme long shot odds given the dramatically greater likelihood of a false positive result than infection itself?  Even for special reassurance?  1 in 2000 seems like high odds one could get a false positive?

Thank you again, Doctors, for the continual great work.
Edward W. Hook M.D.
Edward W. Hook M.D.
22 months ago
I'm sorry that you continue to worry about this exposure.  We provided a single answer to each of your last two posts.  I will also provide a single answer one more time and then future repetitive, overthought anxiety-driven questions will be deleted without comment.  Brief answers:

1.  Yes, the math would suggest one in billions.  There is no meaningful difference however between one in hundreds of billions and one in billions.  both are effectively zero. You need to get over this.
2.  High/small - hard to say.  Clearly this risk for false positives is probably higher than your risk of a true positive but it is still very, very small.  The precise risk of false positive HIV test results is difficult to estimate precisely because studies of these tests enroll "only" thousands or rarely tens of thousands of persons.  Further, with since initial positive tests are typically verified using a second, unrelated test for verification, the likelihood of having false positive HIV tests, even if one test is falsely positive are vanishingly low.  As you point out however, we do recommend against excessive testing in part because it is a waste of resources even more than there is a tiny risk of a false positive test.  

End of response.  EWH
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