The Hepatitis B Surface Antigen (HBsAg) test is a blood test that checks for the presence of hepatitis B surface antigen, a protein found on the outside of the hepatitis B virus (HBV). If HBsAg is detected, it usually means the virus is currently in your body and you have an active hepatitis B infection (either a recent/acute infection or a long-term/chronic infection).

This test is commonly used as a first-line screening test for “hep B” and is often ordered along with other hepatitis B blood tests (such as anti-HBs, anti-HBc, and sometimes HBV DNA) to better understand whether an infection is new, chronic, or resolved and whether you may be contagious.

What results can mean (plain language):

Negative (HBsAg not detected): Usually means you do not have an active hepatitis B infection at the time of testing. However, a negative result does not always rule out very early infection (the “window period”) or rare testing scenarios—your clinician may order additional tests if risk is high or symptoms suggest hepatitis.

Positive (HBsAg detected): Usually means you currently have hepatitis B and can potentially spread HBV to others through blood or certain body fluids. If HBsAg stays positive for more than 6 months, it typically indicates chronic hepatitis B. Your healthcare provider may recommend follow-up testing (e.g., HBV DNA/viral load, HBeAg, and liver enzymes like ALT/AST) to evaluate infectivity and liver inflammation or damage.

Doctors order an HBsAg test to screen for active hepatitis B, to evaluate possible liver inflammation, and to help protect close contacts and newborns from exposure. It may be recommended even if you feel well, because hepatitis B can be silent for years while still affecting the liver.

Common reasons and situations include:

Symptoms of hepatitis or liver problems: fatigue, nausea, vomiting, loss of appetite, abdominal pain (especially on the right side), dark urine, pale stools, fever, joint aches, or jaundice (yellowing of the skin or eyes).

Abnormal liver tests: elevated ALT/AST or other findings suggesting liver inflammation, prompting evaluation for causes such as hepatitis B.

Screening due to risk factors or possible exposure: unprotected sex with a new partner, multiple sex partners, known exposure to someone with hepatitis B, sharing needles or injection equipment, needlestick injuries, or sharing items that may have blood (razors, toothbrushes).

Pregnancy screening: routinely performed during pregnancy to reduce the risk of passing hepatitis B to the baby at birth; a positive result leads to important steps to protect the newborn.

Before certain medications or treatments: prior to immunosuppressive therapy, chemotherapy, biologics, or organ/stem cell transplant, because hepatitis B can reactivate when the immune system is weakened.

People with higher prevalence risk: those born in or who lived in regions where hepatitis B is more common; people living with HIV; people on hemodialysis; or those with chronic liver disease.

Follow-up and monitoring: to confirm ongoing infection, assess chronic hepatitis B status, and guide next steps in evaluation and care.

• Acute hepatitis B infection
• Chronic hepatitis B infection
• HBV reactivation (e.g., during chemotherapy or immunosuppressive therapy)
• Hepatitis (liver inflammation) and hepatitis-related jaundice
• Cirrhosis (scarring of the liver) related to chronic hepatitis B
• Hepatocellular carcinoma (primary liver cancer) associated with chronic hepatitis B

• Screening for active hepatitis B (“hep B”) infection
• Protecting liver health by identifying infection early and preventing long-term liver damage
• Reducing transmission risk to partners, household contacts, and the community
• Supporting healthy pregnancy and preventing mother-to-baby hepatitis B transmission
• Guiding vaccination and immunity decisions when paired with other hepatitis B tests
• Pre-treatment safety screening before immunosuppressive medications, biologics, or chemotherapy
• Monitoring chronic hepatitis B care planning alongside liver function tests and HBV viral load