p24 Antigen

The p24 antigen test detects actual HIV viral protein in blood. The test is generally positive from about one week to 3-4 weeks after infection with HIV. The p24 protein cannot be detected until about a week after infection with HIV because it generally takes that long for the virus to become established and multiply to sufficient numbers that they can be detected. About 2-8 weeks after initial exposure, antibodies are produced in response to HIV infection. Once antibodies are produced, the results of the p24 test will usually be negative although the person may be infected with HIV. The antibodies bind to the p24 protein, causing the p24 antigen to no longer be detected in the blood. At that point, however, the HIV antibody test that is most often used for routine screening will be positive. Later in the course of HIV, p24 protein levels again become detectable if the disease is untreated.

The use of this test has declined somewhat, especially with the increase in use of molecular tests to detect early infections (see Viral Load) and to screen blood and blood products. However, it is often used in areas where resources are limited and where molecular tests are not as widely available.

Sometimes this test may be combined with a test for HIV antibodies to increase the likelihood of detecting HIV infection in the early stages.

A blood sample is obtained by inserting a needle into a vein in the arm.

No test preparation is needed.