p24 Antigen Test

The p24 test identifies actual HIV particles in blood (p24 is a protein "shell" on the surface of HIV). However, the p24 test is generally only 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. The p24 proteins then become undetectable again after sufficient antibodies to HIV have been produced because they bind to the p24 protein and eliminate it from the blood. Once antibodies are produced, the p24 test will be negative even in people who are infected with HIV. At that point, the standard HIV antibody test will be positive. Later in the course of HIV, p24 protein again becomes detectable.

The collection method depends on the type of test kit used. A blood sample can be collected by finger-prick or by taking blood through a needle placed in a vein in your arm.