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Accuracy and timing of HIV PCR Testing

Feb 5, 2001

Can you tell me what you know regarding the accuracy and timing issues associated with HIV PCR testing? I have been told that the minimum time frame is 28 days following the last possible exposure. Additionally, I've read a lot about a high level of false positives associated with this test (are there distinct statistics for this?). I realize that I need to follow up with a 3 and/or 6 month antibody test. For my own sanity, however, I feel I must move forward with the PCR testing.

Response from Mr. Kull

PCR tests are not licensed as tools for diagnosing HIV infection, so there are currently no clear standards set for using PCR tests to detect infection. HIV PCR tests are only approved for monitoring viral load in infected persons. The p24 antigen test is the only licensed tool for detecting infection earlier than the antibody tests (it is used in screening blood donations), but is not as sensitive as PCR testing for viral RNA. The p24 test is more likely to be falsely negative than PCR testing or antibody testing after the window period because it lacks the other tests' sensitivity.

A recent review of literature on primary HIV infection states that PCR tests for viral RNA can detect infection one to three weeks earlier than the standard antibody test (which is about, on average, 3-4 weeks following infection). Virus has been detected in plasma anywhere from 4 to 11 days following infection. While false positives on viral-RNA tests are rare, they are more common than false positives on antibody testing because of the PCR's increased sensitivity and the technical complexity of performing the tests.

PCR tests are most meaningful when a physician is attempting to diagnose symptoms that suggest acute infection (when viral levels should be high). Otherwise, since levels of virus in an infected person are variable based on immune response, viral characteristics, and type of test used, a PCR test used alone is not sufficient. PCR testing should ALWAYS be confirmed with antibody testing after the window period has elapsed.


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