Many HIV tests look for p24 antigen in addition to antibodies to HIV. These are sometimes called fourth-generation tests.
One distinctive HIV antigen is a viral protein called p24, a structural protein that makes up most of the HIV viral core, or 'capsid'. High levels of p24 are present in the blood serum of newly infected individuals during the short period between infection and seroconversion, making p24 antigen assays useful in diagnosing primary HIV infection.
Antibodies to p24 are produced during seroconversion, rendering p24 antigen undetectable after seroconversion in most cases. Therefore, p24 antigen assays are not reliable for diagnosing HIV infection after its very earliest stages. However, HIV infection can be reliably diagnosed earlier with combined antibody/antigen tests than with purely antibody-detecting tests, and fourth-generation antibody/antigen tests are now the standard screening assay in the UK and some other countries.