Question
Hepatitis B surface antigen (HBsAg)
Answer
Hepatitis B surface antigen (HBsAg) is a protein component of the outer envelope of the Hepatitis B virus, and the first serological marker to appear in the blood after HBV infection, typically detectable within a few weeks of exposure, before symptoms even develop.
Significance:
- Indicates current infection, whether acute or chronic — its presence confirms active HBV infection is present.
- Persistence of HBsAg beyond 6 months defines progression to chronic HBV infection.
- Used as the primary screening test for HBV infection in blood donors, pregnant women, and clinical diagnostic work-up of suspected hepatitis.
- Forms the basis of the Hepatitis B vaccine, which contains purified/recombinant HBsAg to stimulate protective anti-HBs antibody without causing infection.
Clearance and its significance: clearance of HBsAg (with subsequent development of anti-HBs) indicates resolution of infection and recovery with immunity; failure to clear HBsAg within 6 months indicates chronic infection, carrying long-term risk of cirrhosis and hepatocellular carcinoma.
Distinguishing from anti-HBs: HBsAg indicates the virus (antigen) is present in the patient; anti-HBs is the protective antibody the host produces against it — the two are essentially mutually exclusive in an individual’s serum at any given time (anti-HBs typically rises only after HBsAg has cleared).