Hepatitis B Surface Antigen (HBsAg)
Hepatitis B surface antigen (HBsAg) is a protein found on the surface of the hepatitis B virus. This blood test looks for that protein to see whether the virus is currently present in your body.
HBsAg is usually the first marker to appear when someone is infected with hepatitis B, a virus that affects the liver. Its presence signals a current infection, which may be recent (acute) or long-standing (chronic). Identifying hepatitis B matters because chronic infection can, over many years, lead to liver damage, and knowing your status allows for monitoring, treatment when appropriate, and steps to protect others.
A positive (reactive) result means the hepatitis B virus is currently present in your body. This could reflect a new (acute) infection or a chronic one. A single positive result usually needs to be confirmed and interpreted alongside other hepatitis B markers, and often repeated after six months, to determine whether the infection is acute or chronic. Your clinician will guide next steps, including further testing and possible referral.
A negative (non-reactive) result means hepatitis B surface antigen was not detected, indicating you likely do not have a current hepatitis B infection. It does not, on its own, tell you whether you have immunity from past infection or vaccination; other markers such as anti-HBs and anti-HBc provide that information.
The expected, normal result is negative (non-reactive). Because hepatitis B status is best understood by looking at several markers together, your clinician will interpret HBsAg alongside anti-HBs and anti-HBc, and confirm any positive result before drawing conclusions.
Sources
- Hepatitis B Surface Antigen (HBsAg) Blood Test: MedlinePlus
- Hepatitis B Questions and Answers for Health Professionals: CDC
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.