What should you know about hepatitis B surface antigen test?

The test looks for hepatitis B surface antigen, a protein associated with the virus. A reactive result suggests current infection and needs interpretation with other markers. A non-reactive result means the marker was not detected in that sample, but it does not by itself show immunity or exclude every early or past-infection pattern.

The test may be included in screening, pregnancy care, assessment after a possible exposure, or evaluation of symptoms. Hepatitis B can be present without symptoms; when symptoms occur, fatigue, nausea, abdominal discomfort, or dark urine may appear. Read about [Hepatitis B](/conditions/hepatitis-b/), [Fatigue](/symptoms/fatigue/), and [Dark Urine](/symptoms/dark-urine/).

A blood sample is used, and your clinician will explain whether any preparation is needed. Keep the full report rather than focusing on one word. Follow-up can include surface antibody and core antibody testing, repeat testing when timing matters, and a [Liver Function Test](/treatments/diagnostics-and-screening/liver-function-test/). Bring your vaccination record and mention possible exposure, pregnancy, symptoms, and medicines. If you are not immune, ask about [Hepatitis B Vaccination](/treatments/preventive-care-and-vaccination/hepatitis-b-vaccination/).

Ask for timely review if testing follows a recent exposure, if you are pregnant, or if immune-suppressing treatment is planned. Seek urgent care for yellow eyes or skin, confusion, repeated vomiting, unusual bleeding, severe drowsiness, or significant abdominal swelling. Do not wait for routine results if these warning signs occur.

Ask why the test was ordered, what the result means with the rest of the panel, whether confirmation or repeat testing is needed, and whether liver monitoring is appropriate. Ask whether household or sexual contacts need testing or vaccination.