P24 antigen is an antigen of the HIV virus, which is often used to test for AIDS. Normal HIV testing is done by checking for antibodies to HIV, including type 1 and type 2 antibodies, which has a slightly longer window period, which can be shortened by applying other reagents along with the P24 antigen test. the P24 antigen appears earlier than the antibodies to the virus, so HIV can be detected earlier, but if the P24 antigen disappears more than 2-3 months later, the sensitivity of detecting HIV is the same, whether the test is done for the P24 antigen or not, because it is the HIV antibody that is tested at that time. However, if the P24 antigen disappears after 2-3 months, the sensitivity of the test is the same with or without the P24 antigen, because it is the HIV antibody that is being tested.