As its name suggests, the p24 antigen test is designed to detect a specific protein from the surface of the virus: the p24 antigen. Antigens stimulate a person’s immune system to make antibodies, which are also proteins, and help the body to fight off the infection. The p24 antigen test may detect the presence of infection more quickly than the antibody tests discussed previously, and it is currently used by blood banks to detect the virus in those who are infected but have not yet developed antibody. It has a very high sensitivity for detecting HIV early in the course of infection, when the level of circulating virus is very high. It is not, however, recommended as a routine screening test for HIV at this point, because for those persons beyond the initial, acute stage of infection, its sensitivity is not as high as that of the ELISA and Western blot assay.
The p24 antigen test is also useful in detecting infection in newborns born to HIV-infected mothers, since these babies may have passively acquired their mothers’ antibody to HIV infection and therefore would test positive on the ELISA and Western blot assay, whether or not they were truly infected. The presence of the p24 antigen in an infant indicates true infection. Some of the tests designed to detect the genetic material of the virus (described later in this section) may be more accurate than this test for the detection of HIV infection in newborns, and they are now used more commonly for testing newborns.
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