The Hepatitis B Surface Antibody test—also called HBsAb or anti-HBs—is a blood test that measures antibodies your immune system makes against the hepatitis B surface antigen (HBsAg). These antibodies are considered protective, meaning they can help prevent hepatitis B infection.

This test is most commonly used to confirm hepatitis B immunity. A positive (reactive) anti-HBs result usually means you are protected because you either:

1) Were vaccinated and developed an adequate Hep B vaccine response, or
2) Had a past hepatitis B infection and recovered, leaving you with natural immunity.

A negative (non-reactive) anti-HBs result usually means you do not have protective immunity and may be susceptible to hepatitis B, especially if you have not been vaccinated or did not respond to the vaccine. Results are often reported as a number (titer) in mIU/mL; many labs consider a level around ≥10 mIU/mL consistent with protective immunity, though your clinician will interpret this based on your history and other hepatitis B labs.

Because anti-HBs is only one part of hepatitis B testing, doctors often interpret it alongside other tests such as HBsAg (to check for current infection) and total anti-HBc (to check for past exposure). The pattern of results helps clarify whether immunity is from vaccination versus prior infection.

Providers order an HBsAb (anti-HBs) test to check whether you are protected against hepatitis B, to confirm vaccine response, or to help interpret hepatitis B status when other results are unclear.

Common reasons this test is done include:

To confirm immunity after vaccination (hepb vaccine response), especially for people who need documented protection, such as:

• Healthcare workers, first responders, and people with occupational blood exposure risk
• People on hemodialysis or with chronic kidney disease
• Individuals with weakened immune systems (e.g., due to certain medications or conditions)
• Infants born to a parent with hepatitis B, and others at higher risk

To evaluate immunity before or during situations with higher exposure risk, including:

• Household or sexual contact with someone who has hepatitis B
• Multiple sexual partners or a history of sexually transmitted infections
• Injection drug use or sharing needles/equipment
• Travel or living in areas where hepatitis B is more common
• People who are incarcerated or in settings with increased exposure risk

As part of hepatitis B screening or a broader liver/infectious disease evaluation. Even though hepatitis B often causes no symptoms at first, testing may be prompted by:

• Abnormal liver enzymes (ALT/AST) on bloodwork
• Symptoms of hepatitis such as fatigue, nausea, abdominal discomfort, dark urine, pale stools, or jaundice (yellowing of skin/eyes)
• Pregnancy screening (typically includes HBsAg; anti-HBs may be used for immunity assessment depending on the situation)

Before certain medications or treatments (e.g., immunosuppressive therapy, chemotherapy, some biologics). In these settings, clinicians may assess hepatitis B status and immunity to reduce the risk of complications related to hepatitis B exposure or reactivation.

• Hepatitis B virus (HBV) infection (acute or chronic)
• Resolved (past) hepatitis B infection with natural immunity
• Lack of immunity to hepatitis B (susceptible/not protected)
• Vaccine non-response or inadequate response to hepatitis B vaccination
• Immunocompromised states affecting antibody production (e.g., due to medications or certain medical conditions)

• Confirming hepatitis B immunity (hbsab/anti-HBs) after vaccination
• Guiding preventive care, including whether a hepatitis B vaccine booster or repeat series may be needed
• Reducing risk of hepatitis B infection in higher-exposure occupations or lifestyles
• Supporting liver health by clarifying hepatitis B status during evaluation of abnormal liver tests
• Preconception and pregnancy planning: ensuring protection against hepatitis B for parent and baby
• Safer initiation of immunosuppressive medications by documenting hepatitis B immunity and status