The HIV antibody test advised by the CDC is the HIV-1/2 antigen/antibody combination immunoassay test. If you test positive for HIV, the CDC advises the following follow-up tests: HIV-1/HIV-2 antibody differentiation immunoassay. An antigen test checks your blood for an HIV antigen, called p24. When you're first infected with HIV, and before your body has a chance to make antibodies to the virus, your blood has a high level of p24. The p24 antigen test is accurate 11 days to 1 month after getting infected.