To Check for Hepatitis B Immunity, Order Hepatitis B Surface Antibody (anti-HBs), Not Antigen
You should order hepatitis B surface antibody (anti-HBs) testing to assess immunity, not hepatitis B surface antigen (HBsAg). These are fundamentally different tests that answer opposite questions about hepatitis B status.
Understanding the Key Distinction
Anti-HBs (antibody) indicates immunity - either from successful vaccination or recovery from past infection - while HBsAg (antigen) indicates active infection 1. The CDC explicitly recommends measuring anti-HBs to assess immune status, with a protective level defined as ≥10 mIU/mL 1.
What Each Test Tells You:
- Hepatitis B surface antibody (anti-HBs): Demonstrates protective immunity when ≥10 mIU/mL 1, 2
- Hepatitis B surface antigen (HBsAg): Indicates current HBV infection (acute or chronic) 1, 3
When to Order Anti-HBs Testing
The CDC recommends post-vaccination serologic testing (anti-HBs) 1-2 months after the final vaccine dose for specific populations 1:
- Healthcare personnel and public safety workers at risk for blood/body fluid exposure 1
- Hemodialysis patients and other immunocompromised persons (HIV-infected, transplant recipients, chemotherapy patients) 1
- Sexual partners of HBsAg-positive persons 1
- Infants born to HBsAg-positive mothers 1
Important Caveat for Previously Vaccinated Individuals:
For healthcare workers with documented complete vaccination during childhood, institutions may choose to measure anti-HBs upon hire, though this is optional 1. Because vaccine-induced antibody wanes over time, testing years after vaccination may not distinguish vaccine nonresponders from responders who have lost detectable antibody but retain immune memory 1.
Interpreting Anti-HBs Results
The protective threshold is anti-HBs ≥10 mIU/mL 1, 2:
- Anti-HBs ≥10 mIU/mL: Indicates adequate immunity; no further action needed for immunocompetent individuals 1, 2
- Anti-HBs <10 mIU/mL: Indicates inadequate response; requires revaccination 1
Management Algorithm for Low or Negative Anti-HBs:
- Administer one booster dose of hepatitis B vaccine 1
- Retest anti-HBs 1-2 months later 1
- If still <10 mIU/mL: Complete a second full 3-dose vaccine series 1
- Retest 1-2 months after final dose of the second series 1
Complete Serologic Profile for Comprehensive Assessment
While anti-HBs alone assesses immunity, a complete hepatitis B panel helps distinguish different infection states 4, 5:
- Anti-HBs positive alone (with negative HBsAg and anti-HBc): Vaccine-derived immunity 4, 5
- Anti-HBs positive with anti-HBc positive (HBsAg negative): Immunity from past natural infection 4, 2
- HBsAg positive: Current infection, regardless of anti-HBs status 3
Common Pitfalls to Avoid
Do not confuse antigen with antibody - ordering HBsAg when you mean to check immunity is a fundamental error that screens for infection rather than protection 1, 3.
Passively acquired anti-HBs (from HBIG or IVIG administration) can produce false interpretation of immune status and may be detected for 3-6 months after administration 1, 4, 6. Always obtain vaccination and treatment history before interpreting results.
Timing matters: Post-vaccination testing should occur 1-2 months after the final vaccine dose using a quantitative method (ELISA) that detects the protective concentration 1, 2.
For immunocompromised patients, particularly those on hemodialysis, annual anti-HBs monitoring with booster doses when levels fall <10 mIU/mL is recommended, as these patients have lower seroconversion rates and may lose immunity more rapidly 1, 2, 3.