HBsAg

HBsAg, or hepatitis B surface antigen, is a protein that forms part of the outer envelope of the hepatitis B virus (HBV). It is produced in excess by infected liver cells and can circulate in the bloodstream as small, non-infectious subviral particles as well as as part of infectious virions. The presence of HBsAg in blood is a key serological marker of current HBV infection.

Detection of HBsAg is used to diagnose active HBV infection. It may be detectable in the acute phase, often before symptoms. The infection is considered chronic if HBsAg remains detectable for more than six months. In people who have completed HBV vaccination, HBsAg is not present and anti-HBs antibodies indicate immunity.

HBV is transmitted through percutaneous or mucosal exposure to infected blood and body fluids. Pregnant mothers can transmit the virus to newborns; sexual contact and sharing needles are common routes. HBeAg status and HBV DNA levels are used to assess infectivity, with HBeAg associated with higher levels of virus replication.

HBsAg is typically detected with immunoassays such as ELISA or chemiluminescent assays and is routinely used in blood donor screening and clinical evaluation. If chronic HBV infection is diagnosed, management focuses on monitoring liver function, HBV DNA, and potential antiviral therapy to reduce viral replication and liver damage. Loss of HBsAg, though achievable, is relatively uncommon and regarded as a functional cure; anti-HBs seroconversion marks acquisition of lasting immunity after clearance or vaccination.

HBsAg biology includes multiple surface protein variants (small, middle, and large surface antigens) that assemble into the viral envelope. The marker alone does not indicate infectivity; assessment often requires HBV DNA quantification and clinical context. Overall, HBsAg testing remains a cornerstone of HBV diagnosis and public health screening.