I had an hiv test 8 1/2 weeks after a possibly risky act, it came back negative. My question is : what percentage of people would have the anti-bodies present in their blood 8 weeks from time of infection? Should I have the test again at 6 months? Also, I have heard of a test that measures the actual virus, not the anti-bodies, the "PCR" test. Is it true there is no window period with this test, and could my personal doctor run this test and how much would it be.

In regard to how accurate a test is in 8 1/2 weeks, it's important to understand what the "window Period" is for an antibody test to show positive. In terms of window periods, there are 4 time points to remember:

25 days:AVERAGE window period 3 months:USUAL window period (most cases but not all) 6 months:FULL window period (>99% accuracy) >6 months:EXTREMELY RARE window period beyond 6 months

So a negative test at 8 1/2 weeks certainly looks promising, but it is not conclusive. Only a negative test at 6 months is considered conclusive that you are not infected (with more than 99% accuracy). Remember that a 6 month test (or anytime after that) is considered the most accurate test to determine whether a person is infected or not, so you should consider getting re-tested at the 6 month timepoint. There is a PCR test (Polymerase Chain Reaction) that measures the virus itself and one type of PCR test (Qualitative DNA) can be used for diagnosis of HIV. The window period for this test is somewhere from 1-2 months (depending which lab or test manufacturer you ask). However I must point out that this test is NOT designed for testing adults for general screening purposes like yours. It's primary use is for diagnosing newborn babies less than 18 months of age (when antibody tests would not be reliable), and for research purposes. These tests can only be specially ordered through your private physician, and can be very expensive (several hundred dollars). These tests are very difficult for labs to perform (hence the high cost), and only certain labs have the facilities to run them. Again, let me emphasize here that these tests are not designed for general screening purposes--the antibody tests are best designed for that. There are other types of PCR tests used to measure "viral load" in persons already known to have HIV, but these are different from the PCR tests I'm talking about here. My best suggestion for you now is to follow-up on your 8 1/2 week test with one at 6 months. That would be the best way to determine whether you've been infected or not.

If you have further questions, please feel free to call the Centers for Disease Control at 1.800.232.4636 (Nationwide).

Rick Sowadsky MSPH CDS