Hepatitis B virus surface antigen (HBsAg), is the shell substance of hepatitis B virus. HBsAg positivity indicates the presence of intact hepatitis B virus particles in the body, indicating that the body is infected with hepatitis B virus. The intact hepatitis B virus is a substance consisting of an envelope and a nucleocapsid, with the envelope containing HBsAg, glycoproteins and other components. There is a protein on the surface of the virus that can be used as an antigen to stimulate the body’s immune system to produce hepatitis B surface antibodies, so it is a marker of hepatitis B virus infection in the body, the first of the five items of hepatitis B (i.e. hepatitis B two and a half), if the test shows a positive “surface antigen”, it means that the body is infected with hepatitis B virus. However, the surface antigen does not reflect the replication status of the hepatitis B virus in the body, so it is not an indicator of hepatitis B infectivity. HBsAg positive people may be latent hepatitis B, asymptomatic hepatitis B virus carriers, or chronic hepatitis B patients. If the hepatitis B half test shows positive HBsAg, do not be alarmed, but further check liver function and hepatitis B virus nucleic acid (HBVDNA), if HBVDNA is positive, it means that there is hepatitis B virus replication and infectious; if there is both abnormal liver function and positive HBVDNA, it indicates hepatitis virus activity, and antiviral treatment should be carried out in time. In summary, a positive hepatitis B virus surface antigen indicates hepatitis B virus infection in the body, but it is not yet possible to determine whether it is infectious and should be analyzed together with other serological test results. Therefore, do not panic and seek prompt medical attention at the hospital.