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PCR Question
Feb 12, 2002

Mr. Kull: The CDC states that this test can pick up the HIV virus in someone only recently infected. My question is how accurate is the test say at 16 weeks, and what does the test look for, that the antibody test does not?

Response from Mr. Kull

An HIV antibody test at 16 weeks will be more accurate than a PCR test at 16 weeks, and antibody testing at that point makes more sense. A negative PCR (no viral RNA or DNA detected) at 16 weeks most likely means that you are not infected, but an antibody test result is preferable.

PCR (polymerase chain reaction) tests are used to detect the presence of HIV-RNA or DNA in a person's blood. Essentially, the PCR tests are looking for evidence of virus itself and not antibodies. These tests are not FDA approved for diagnostic purposes because of higher rates of false positives when compared to the much more specific antibody tests. 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 uncommon, 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. So, unless PCR testing is absolutely necessary, antibody testing is recommended.

RMK



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