Important note: Information in this article was accurate in 2004. The state of the art may have changed since the publication date. I had unprotected sex just over 5 weeks ago. I had a an antibody HIV test 33 days after possible infection which was negative. I am thinking about having a PCR test. If I take a PCR test which comes out negative then will this combined with the negative antibody result after 33 days be a 100% certain result?
Rodger MacArthur, M.D.
Wayne State University
Division of Infectious Diseases
Only the HIV antibody test is approved for use in determining HIV status in the situation described. The reverse transcriptase polymerase chain reaction (RT PCR) test can be negative in some persons who are HIV antibody positive, although not typically in newly infected persons. However, the RT PCR test can appear to be falsely positive on occasion, with apparent low levels (< 2000 copies/ml) of HIV RNA detected. This latter situation occurs most often because the tests are batched. That is, specimens from many individuals are run sequentially on the same day, which occasionally results in contamination of the equipment with another individual's HIV RNA.
The HIV antibody test may turn positive as early as about 30 days after exposure. It takes that long or longer for the body to produce detectable amounts of antibody to HIV. Consequently, it is advisable to be tested at additional time points following a risk exposure. Most authorities recommend baseline testing (within 7 days of exposure), and 30, 90, and 180 days after exposure. If an individual is repeatedly negative for at least 180 days following exposure, and assuming no additional risk exposures in that time interval, it can safely be assumed that seroconversion (from HIV negative to HIV positive) will not occur.Finally, it should be noted that false positive HIV antibody tests do occur on occasion, especially in individuals from low risk populations. That is, the false positive rate of the test increases as the a priori (pre-test) probability of being positive decreases. False negative results occur only in the setting of recent exposures, prior to the time it takes for the body to produce antibodies to HIV. Anyone testing positive on the HIV antibody test should seek medical advice from a physician experienced in HIV medicine.
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