Hepatitis B Surface Antigen (HBsAg) Test

HBsAg, or Hepatitis B Surface Antigen, is a protein found on the surface of the Hepatitis B virus, and its presence in your blood is the earliest and most direct marker that the virus is actively present in your body.

A positive HBsAg result appears well before symptoms typically develop, which is what makes this test so valuable for early detection, screening, and preventing further transmission.

If HBsAg remains detectable for more than six months, it indicates the infection has become chronic rather than resolving on its own, a distinction that significantly changes long-term monitoring and treatment approach.

Because Hepatitis B can be transmitted through blood, from mother to child during birth, and through sexual contact, HBsAg screening is a standard part of prenatal care, blood donation screening, and pre-surgical workups in many healthcare systems.