The HIV p24 antigen test
The HIV p24 antigen test, the most widely available of the two, is designed to detect a protein (the p24 protein) associated with HIV. The p24 antigen test can detect the p24 protein on average 10 to 14 days after infection with HIV.2,3,11,12 One drawback of this test is that levels of the p24 protein peak at around three to four weeks after exposure to HIV and are usually not detectable after five to six weeks (and sometimes even earlier). A positive p24 test means that someone is HIV-positive. However, a negative p24 test can mean three things:
- the person is HIV-negative
- the person is HIV-positive but that the test could not detect the p24 protein because the person was infected more than four to six weeks earlier
- the levels of p24 antigen are too low to be detectable with current technologies.
The most advanced tests combine a p24 antigen test and an antibody test. These tests are seen as beneficial because they combine the early detection abilities of the p24 antigen test with the accuracy of the newer antibody tests. It should be noted that a rapid (point-of-care) version of these tests is not yet available.