Blood and lab tests
What Does HBsAg Mean on a Blood Test?
HBsAg · say it: aych-bee-ess-ay-jeeAlso written: hepatitis B surface antigen, Australia antigen, HBs antigen, hep B surface antigen
Short answer
HBsAg is a blood test that looks for a protein from the surface of the hepatitis B virus.
What it means
HBsAg stands for hepatitis B surface antigen. An antigen is a part of a germ that the immune system can recognise. This particular protein sits on the outer coat of the hepatitis B virus. When HBsAg is found in the blood, it means hepatitis B virus is present in the body at the time of the test.
HBsAg is the main screening test for hepatitis B. It is done in pregnancy, before some treatments, before blood donation, and when someone has liver test changes or a risk of exposure. If it is positive, doctors usually do more tests to work out whether the infection is recent or long-standing and how active it is. If HBsAg stays positive for more than six months, the infection is called chronic hepatitis B.
HBsAg is easy to confuse with anti-HBs (also written HBsAb). HBsAg is the virus protein. Anti-HBs is the antibody the body makes against it, which shows protection from vaccination or a past infection that has cleared.
Where you may see it
- Antenatal or pregnancy blood test results
- Hepatitis screening panels, often printed with anti-HBs and anti-HBc
- Pre-surgery, pre-chemotherapy or pre-dialysis blood tests
- Blood donation and employment health check results
📄 How it looks on a report
HBsAg: Reactive. Confirmatory testing to follow.
🙅 What it does not mean
- A positive HBsAg does not tell you how long you have had hepatitis B or how much liver damage there is. Further blood tests and sometimes a liver scan are used for that.
- A negative HBsAg does not mean you are protected against hepatitis B. Protection is shown by anti-HBs, not by HBsAg.
- "Reactive" on a screening report is not always the final answer. Labs often confirm a reactive result before reporting it as positive.
🗒️ Questions to ask your doctor
🧾 Reports where you will see “HBsAg”
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🔗 Read more from trusted sources
- “HBsAg” on MedlinePlus ↗
US National Library of Medicine
- MedlinePlus lab test guides ↗
US National Library of Medicine
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General information about a medical word, not advice about your own results. Ask the clinician who ordered the test what it means for you.