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Reasons for False Positive results

Aug 13, 2011

Dear Doctor, I just read that you said over-testing for HIV can lead to a false positive. Is that really true and why/how? Also, what happens if you test indeterminate, then positive, then negative in a 5 week period? Are there other diseases that can cause false positives that should be checked? Epstein-Barr Virus, etc.? Should wait until the 3 month mark before retesting to be 100% certain? I've known my GP for 10 years.

Thank you!

Response from Dr. Frascino


The reason over-testing could lead to a false positive is pure statistics. It's like buying lottery tickets with a false-positive test being analogous to winning the megabucks jackpot. The vast majority of lottery tickets don't win, just as the vast majority of HIV tests don't result in false-positive results. However, the more lottery tickets you buy, the chance of hitting a winning ticket increases. Same thing with unwarranted over-testing. Your chances of getting a "false positive" increases with the number of unwarranted tests.

Once someone tests positive for HIV, more sophisticated tests are run that would expose a false-positive result. You don't need to check for specific diseases, such as Epstein-Barr virus, etc.

Regarding HIV-diagnostic testing, a negative test taken before the three-month mark is not considered to be definitive.

If someone tests HIV positive on a screening ELISA/EIA and confirmatory Western Blot test, he should be referred to an HIV specialist who will confirm the diagnosis and run more sophisticated tests to check for viral activity (viral load) and immune damage from HIV (CD4 count, CD4%). If someone tests HIV negative on screening ELISA/EIA tests outside the window period, they would be considered definitively negative and no further testing would be warranted.

Dr. Bob

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