The HBsAg ELISA test identifies the hepatitis B surface antigen (HBsAg), a protein found on the outer surface of the virus. Detecting this antigen means the virus is present in the bloodstream and the person may be capable of transmitting the infection.

The test is commonly performed using the Enzyme-Linked Immunosorbent Assay (ELISA), which is a sensitive technique for detecting specific antigens in blood samples.