An HIV antigen/antibody test looks for both HIV antibodies and the p24 antigen. The p24 antigen is part of HIV and may become detectable before HIV antibodies develop. Because of this, antigen/antibody tests can detect some HIV infections earlier than antibody-only tests.
The window period depends on the type of antigen/antibody test and the sample used. According to the CDC, a laboratory antigen/antibody test using blood from a vein can usually detect HIV 18 to 45 days after exposure. An antigen/antibody test using blood from a finger stick can usually detect HIV 18 to 90 days after exposure.
Testing too early can still produce a non-reactive result even when HIV is present. If you recently had a possible exposure, read our When to test for HIV? page for more information about HIV test window periods.
For additional information about antigen/antibody testing, see the CDC HIV testing guidance.
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