Hepatitis B surface antigen (HBsAg)
Also called: Hepatitis B surface antigen, HBsAg, HBs antigen, Australia antigen
A protein from the outer coat of the hepatitis B virus. Finding it in the blood means an active hepatitis B infection is present — the virus is there and potentially transmissible. It is the main test used to screen for and diagnose hepatitis B, and to check whether an infection has cleared.
Published: Updated:
Have the whole report? Upload it - we'll explain every value on it
What does Hepatitis B surface antigen (HBsAg) measure?
Hepatitis B surface antigen is a protein from the surface coat of the hepatitis B virus. When the virus is actively infecting the liver, this antigen circulates in the blood, so detecting it is the cornerstone test for diagnosing hepatitis B. A positive (reactive) result means an active infection — either a recent (acute) one or a long-standing (chronic) one — and that the person can pass the virus to others through blood or bodily fluids. A negative result means no active infection is present. HBsAg is used to screen groups at higher risk, to test in pregnancy (so a baby can be protected at birth), to investigate abnormal liver tests or symptoms, and to check blood donations. Its disappearance over time is one of the signs that an acute infection has cleared. Importantly, it is read alongside other hepatitis B markers — the core antibody and surface antibody — which together tell apart current infection, past cleared infection, and immunity from vaccination. This is a supportive, non-judgemental test that opens the door to effective care.
What does a positive Hepatitis B surface antigen (HBsAg) result mean?
- Active hepatitis B infection — a positive (reactive) result means the virus is present; the person is infectious.
- Acute hepatitis B — a recent infection, often with the core IgM antibody also positive.
- Chronic hepatitis B — HBsAg still present after six months means a long-standing infection needing follow-up.
- Read with other markers — the core and surface antibodies distinguish current infection, past infection and vaccine immunity.
Positive Hepatitis B surface antigen (HBsAg) result: full guide
What does a negative Hepatitis B surface antigen (HBsAg) result mean?
- Negative (non-reactive) — the expected result; no active hepatitis B infection.
- After a cleared infection or from vaccination — HBsAg is negative; immunity is shown instead by the surface antibody.
Negative Hepatitis B surface antigen (HBsAg) result: full guide
When to discuss with a doctor
A positive HBsAg means an active hepatitis B infection and should always be assessed by a doctor — it is not a cause for alarm or shame, but the starting point for straightforward, effective care. The next steps are further blood tests (the other hepatitis B markers, viral load, and liver function) to work out whether the infection is acute or chronic and how active it is, plus advice on protecting close contacts through vaccination. Many people with chronic hepatitis B stay well and simply need monitoring; others benefit from antiviral treatment that controls the virus very effectively. In pregnancy, a positive result lets the baby be protected at birth. A negative result is reassuring. Mediora.AI surfaces an HBsAg result alongside the other hepatitis markers and liver tests so the pattern is visible; interpreting it and planning care belong with your doctor.
Related markers
Commonly confused with
- Hepatitis B surface antibody (anti-HBs) The antigen is a piece of the virus itself: finding it means the infection is present. The antibody is your immune response: finding it usually means you are protected, after vaccination or a past infection.