What does a reactive Hepatitis B surface antibody (HBsAb) test result indicate and what are the implications for treatment and management?

Interpretation of Reactive Hepatitis B Surface Antibody (HBsAb)

A reactive Hepatitis B surface antibody (HBsAb) test indicates immunity to Hepatitis B virus (HBV) infection, either from recovery after natural infection or from vaccination. 1

Detailed Interpretation

The interpretation of a reactive HBsAb depends on the results of other HBV serological markers:

Possible Scenarios

1. HBsAb positive + HBsAg negative + HBcAb positive

   • Indicates recovery from past HBV infection and natural immunity
   • Patient has cleared the virus and developed protective antibodies
   • No specific treatment required unless immunosuppressive therapy is planned 1, 2

2. HBsAb positive + HBsAg negative + HBcAb negative

   • Indicates immunity from successful vaccination
   • No evidence of current or past infection
   • No further management needed 1, 2

3. HBsAb positive + HBsAg positive

   • Rare but possible scenario
   • May indicate:
     • Viral mutations in the S gene
     • Immune complex formation
     • Transition phase during seroconversion
   • Associated with increased risk of progressive liver disease 3
   • Requires specialist consultation

Clinical Implications

For Individuals with Natural Immunity (HBsAb+/HBcAb+)

• Generally protected from future HBV infection
• Risk of reactivation exists if immunosuppressed, particularly with:
  • Anti-CD20 monoclonal antibodies (e.g., rituximab)
  • Stem cell transplantation
  • High-dose corticosteroids
  • TNF inhibitors 1, 2

For Individuals with Vaccine-Induced Immunity (HBsAb+/HBcAb-)

• Protected from HBV infection
• No risk of reactivation as there is no latent virus 1
• Protection is considered adequate when HBsAb levels are ≥10 mIU/mL 1

Management Considerations

No Specific Management Required When:

• Patient has vaccine-induced immunity
• Patient has natural immunity and is not undergoing immunosuppressive therapy

Management Required When:

• Immunosuppressive therapy is planned for patients with natural immunity (HBsAb+/HBcAb+):

  • For high-risk regimens (anti-CD20 agents, stem cell transplantation): Consider prophylactic antiviral therapy
  • For moderate-risk regimens: Either close monitoring or prophylactic antivirals
  • Continue antivirals for at least 6-12 months after completing immunosuppressive therapy 1, 2

• Organ transplantation considerations:

  • Recipients with reactive HBsAb (≥10 IU/mL) may receive organs from HBcAb+ donors with lower risk
  • HBsAb+ recipients of HBsAg+ organs still require post-transplant prophylaxis 1

Potential Pitfalls

• False positive HBsAb:

  • Recent administration of intravenous immunoglobulin (IVIG) can cause passive transfer of antibodies 4
  • Does not indicate true immunity

• Waning immunity:

  • HBsAb levels may decrease over time after vaccination
  • Booster doses may be needed in certain high-risk situations 1

• Isolated HBcAb positivity:

  • When HBsAb is negative but HBcAb is positive, this may represent:
    • "Window period" of acute infection
    • Remote resolved infection with waning HBsAb
    • Occult HBV infection
    • False positive result 5

Conclusion

A reactive HBsAb test generally indicates good news - either recovery from past infection or successful vaccination against HBV. The presence of HBsAb is typically protective against future HBV infection, though this protection may be compromised in immunosuppressed states, particularly for those with natural immunity (HBsAb+/HBcAb+).