What Does a Reactive Hepatitis B Surface Antibody (HBsAb) Mean?
A reactive (positive) hepatitis B surface antibody indicates immunity to hepatitis B virus, either from past natural infection that has resolved or from successful vaccination. 1
Determining the Source of Immunity
To understand whether immunity came from vaccination versus natural infection, you must examine the complete hepatitis B panel:
If HBsAg is negative, anti-HBc is negative, and anti-HBs is positive: This pattern indicates vaccine-derived immunity rather than natural infection. 2 The patient received hepatitis B vaccination and mounted a successful immune response.
If HBsAg is negative, anti-HBc is positive, and anti-HBs is positive: This pattern indicates immunity from past natural infection that has resolved. 1 The presence of anti-HBc confirms prior exposure to the virus itself, not just the vaccine antigen.
Clinical Implications of Reactive Anti-HBs
The patient is protected against hepatitis B infection and does not require vaccination. 2 Key points include:
The patient cannot transmit HBV to others and is not infectious. 2
Anti-HBs levels ≥10 mIU/mL generally indicate protective immunity against HBV infection. 2
Immunocompetent individuals typically maintain protection even as antibody levels decline over time, so routine booster doses are not recommended for those with documented seroconversion after vaccination. 2
Important Clinical Distinctions Based on Anti-HBc Status
The presence or absence of anti-HBc alongside reactive anti-HBs has critical implications for patients requiring immunosuppression:
Patients with anti-HBs positive but anti-HBc negative (vaccine-derived immunity) remain protected against HBV reactivation during immunosuppressive therapy because they have no latent virus. 2
Patients with both anti-HBs and anti-HBc positive (past natural infection) carry a 3-45% risk of HBV reactivation with high-risk immunosuppression, particularly with anti-CD20 monoclonal antibodies like rituximab, and require antiviral prophylaxis. 3
Special Populations Requiring Monitoring
Dialysis patients with vaccine-derived immunity require annual anti-HBs screening to monitor for potential loss of immunity, with booster doses recommended when levels fall below 10 mIU/mL. 2, 3
Transplant recipients should have anti-HBs titers checked periodically as they may decline more rapidly in this population. 2
Common Pitfalls to Avoid
Rare false-positive anti-HBs results can occur after administration of intravenous immunoglobulin or recent transfusion with blood products containing passive antibodies. 1, 4 If the clinical context suggests recent passive antibody transfer, repeat testing after an appropriate interval is warranted.
During the serologic "window period" of acute HBV infection, HBsAg may disappear before anti-HBs becomes detectable; during this time, HBc IgM antibody is the only detectable marker. 1 Do not assume immunity based solely on timing if acute infection is suspected.
In most patients with self-limited acute HBV infection, HBsAg and anti-HBs are not detectable simultaneously in serum or plasma. 1 The rare simultaneous presence may indicate unusual serologic patterns requiring further investigation.