What Is Hepatitis B Surface Antigen (HBsAg)?

What it is

HBsAg is a protein on the surface of the hepatitis B virus. It is the earliest marker to appear after HBV infection.

Why it matters

A positive HBsAg indicates HBV infection (acute or chronic). It is the core marker for blood-bank and antenatal screening.

Specimen

Whole blood / serum / plasma

Interpretation

Positive = HBsAg detected (active HBV infection, acute or chronic).

Limitations

Does not distinguish acute from chronic; correlate with other HBV markers.

Related test: HBsAg Rapid Test — specimen: WB / S / P