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PCR or ELISA TESTING?
Feb 28, 2002

Hi, if one took a pcr test almost a year and a half after exposure(unprotected vag. inter.) with no further exposures would that be conclusive or would the elisa test taken at the same time be conclusive or should one take the p24antigen test? If presence of virus found on pcr would a elisa test still be done? What if elisa is still negative but pcr positive presence for virus. What would be done to confirm? Thank you and GOD Bless. This is a very informative site that really helps inform. :)

Response from Mr. Kull

If you are interested in determining your HIV status six or more months after exposure, HIV antibody testing (ELISA/Western Blot) is your BEST option. A negative PCR (no viral RNA or DNA detected) at 1.5 years most likely means that you are not infected, but an antibody screening is preferable.

PCR (polymerase chain reaction) tests are used to detect the presence of HIV-RNA or DNA in a person's blood. Essentially, 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. So neither PCR or p24 is preferable in your case.

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 (recent) 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 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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