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Alternative and faster HIV testing
Nov 7, 1996

I've read about a new HIV test (PCR or p24) that can detect HIV infection within 6 weeks. I'm curious as to knonw what the "appropriate" window period is to get tested with maximum accuracy on this test and how it relates to the regular HIV tests. Are the PCR and p24 tests the same thing?

Response from Mr. Sowadsky

Thank you for your questions. There are a number of tests that can be used to diagnose HIV infection. Let me review each of these tests, and when they normally turn positive after infection.

By far, the most common are the antibody tests. These are also the easiest for labs to perform, and the least expensive. In regard to the accuracy of the antibody tests:

The AVERAGE period of time that an infected person will show positive on the test is 25 days. This is an average, so not all people will test positive by this point in time.

The USUAL period of time that an infected person will show positive on the test is 3 months. This means that most (but not all) infected people will show positive on the test by this time.

The MAXIMUM period of time that an infected person will show positive on the test is 6 months. By this point in time, more than 99% of infected persons will show positive on the test. This is as accurate as any test in medicine could ever be.

For the most accurate antibody test result, you must wait 6 months after your last possible exposure to the virus (or anytime afterward). At 6 months, the antibody tests are more than 99% accurate. If you get tested before the 6 month waiting period, you could have the infection but the test won't pick it up.

As far as the p-24 antigen test is concerned.........

The p-24 antigen tests look for a protein of the virus, rather than antibodies to the virus. The p-24 antigen tests will show positive, on average, 6 days before the antibody tests will. When the antibody tests turn positive, the antigen tests will often revert back to a negative result. The p-24 antigen test alone CANNOT be used to diagnose HIV infection. It must be used alongside another test for HIV. The most common use of this test is in the screening of the American blood supply; in this case, it's used IN ADDITION TO antibody screening of the blood supply.

As far as the Diagnostic PCR test is concerned.........

In regard to the PCR tests, these look for the genetic material of the virus itself. This is a completely different test than the p-24 antigen test. The DNA Qualitative PCR can be used for diagnostic purposes, but there are limitations to it's use. Although it can often pick up an infection by about 1 month after infection, it will not always pick up an infection. It is NOT designed to do routine screening in adults. It's use is presently limited to:

Research applications, where the test can be repeated multiple times to rule out inaccuracies.

Testing in newborns less than 18 months of age where antibody tests are not reliable.

Testing in persons with other diseases of the immune system where antibody tests may not be reliable.

Other UNUSUAL circumstances.

Because PCR tests can sometimes give false positive and false negative results, they are often used in conjunction with other HIV tests to verify their results. As an alternative, when we do Qualitative Diagnostic PCR's, we can also do them more than once, just in case the first one did not accurately pick up the infection. We very rarely do just 1 PCR alone. We usually do more than one test under most circumstances (or in conjunction with other tests) for the most accurate testing possible.

This test is very difficult for labs to perform, hence it's high cost. The more difficult the test, the more the chances for innacuracies. But it's a great test for labs that have a lot of experience with it. Usually only regional reference labs will do this test, but not the vast majority of clinical labs.

Some persons simply don't want to wait 6 months for an antibody test, and therefore ask their physicians to do a PCR test instead. Doing these tests is clinically not necessary in the vast majority of these cases, and only needlessly increases health care costs (these tests can cost several hundred dollars). In the vast majority of cases, antibody tests are all an adult needs. If a person can't cope with the 6 month waiting period, counseling is often a better option than PCR testing.

I hope this clears up the confusion regarding HIV testing. In the vast majority of the population, HIV antibody testing is often the easiest, and most appropriate test to perform. If you have any further questions, please feel free to call the Centers for Disease Control at 1.800.232.4636 (Nationwide). Rick Sowadsky MSPH CDS



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