p24 antigen testing may be used to help diagnose early HIV infection. Levels of p24 antigen increase significantly at about one to three weeks after initial infection. It is during this time frame before HIV antibody is produced when the p24 test is useful in helping to diagnose infection. About 2-8 weeks after exposure, antibodies to HIV are produced and remain detectable in response to the infection, making the HIV antibody test the most useful assay to diagnose an infection.
If the HIV antibody test is negative and you have had a recent exposure to HIV, a p24 test may be ordered to detect the infection. However, this test is not ordered as frequently as it once was since tests that can detect HIV RNA early in infections (HIV viral load) have become more widely available. Tests for HIV antibody are the most commonly ordered screening test.
A positive result means that you are infected with HIV.
A negative result may mean that you are not infected with HIV or that the level of p24 is below the detectable limits of the test. If you suspect that you have been exposed or if you are at an increased risk, repeat testing or screening with a different test, such as the HIV antibody test, is recommended.
The p24 test is one of the earlier tests developed to detect HIV infection. Because p24 is only detectable during a short window of time, its utility is limited. However, this test can still be used when other tests are unavailable.
This article was last reviewed on March 15, 2009.
This page was last modified on April 8, 2009.
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