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Interpreting HIV and testing statistics
Oct 12, 2008

Greetings. I recently tested HIV+ on a rapid blood test, and am awaiting Western Blot results. Needless to say, I'm extremely worried, almost resigned. I've been reading about testing statistics and through questions on this forum, and have a few questions.

Firstly, I notice that a lot of people have questions about indeterminate results, or results on separate tests that bring back different results. I notice that in some answers, unless tests are conclusively negative or positive, that diagnosis can include risk assessment.

Well, I can pinpoint my exposure, as I had only one sexual encounter within the window period. The man I was with was rubbing and probing around my anus without a condom, inserted about a quarter of the way in, and then I stopped him and said he needed to use a condom, and we didn't even have oral sex. So, this might be seen as risky as unprotected anal receptive sex, as I was exposed to his precum, but it did not last long, he did not insert all the way, and did not ejaculate in me. This is the closest I've had to unsafe sex in years, and at the time, considered it foreplay and didn't worry. Two weeks later, I had classic sero-conversion sickness for 5 days, and five weeks after the exposure, tested preliminary positive.

Now, I am almost sadly sure that this means I am HIV+, just because the timing of the positive result is too perfect. However, the day I got sick was the same day that almost everyone I knew felt feverish and achy due to a sudden drop in temperature (perhaps a "bug" that was going around).

As I've read, ELISA tests are 99.9% sensitive and 99.8% specific, which means there's only a 3 in 1000 chance that I got a false positive. But then I read about prevalence at the testing site (i.e. I went to a supposedly low prevalence site, so let's say there will be 20 true positives and 3 false positives out of 1000 tests).

And of course, all I've been inundated with in education, HIV campaigns, and even on this site, is that in the risk assessment of my particular case, since I have not had unprotected sex otherwise . . . that I might be considered in a lower risk group. I've read over and over that transmission rates of receptive anal unprotected sex are about 1% (i.e. even if you're having risky sex it might take multiple exposures).

So I'm confounded by the fact that I may have gotten HIV by a single half-assed (literally) exposure, despite making such a "successful" effort to stay away from the drug scene, be very healthy, very direct and open in my communication with sexual partners, and vigilant about safe sex.

Obviously, I want to have hope, despite the statistics. I've read about quite a few false positive stories here, and I secretly *want* to hear the rapid blood test might be unreliable, that I might even have mono or just had a regular flu. I'm not finding my help on the web as I've found mostly outdated articles or answers that are too general. So, if you would be so kind, I have some specific questions.

- Would you say that the .3% false positive ELISA rate is accurate? If so, does this statistic mean that the .3% *includes* human error and all the possible complications like Epstein Barr and pregnancy or a recent immune system event that can cause a false positive, or does it mean that on top of that, the test itself, if performed perfectly, has a .3% fail rate?

- When it is said that the estimated rate of transmission via unprotected receptive anal sex is 1%, does this mean that if you are actually exposed to HIV, you will get it 1 out of 100 times; or does it mean that *real* exposure actually doesn't occur 99 out these 100 times? (In addition, can an actual positive status be result of "cumulative" exposure?) If it's the former, as I think we are lead to believe when reading statistics, then wouldn't my luck just be truly awful and almost defy the statistics?

- I know that I just must wait for my Western Blot confirmation results. But in that you've given your assessment based on risk factors, and with your knowledge of testing -- would you say I have any reason to hope that my preliminary test is a false positive?

Thanks!

Response from Dr. Holodniy

You need to wait for the western blot test results.



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