What does a hepatitis B (HB) surface antibody level of less than 5 International Units per milliliter (IU/mL) indicate?

Interpretation of Hepatitis B Surface Antibody < 5 IU/mL

A hepatitis B surface antibody (anti-HBs) level less than 5 IU/mL indicates lack of protective immunity against hepatitis B virus infection.

Understanding Anti-HBs Levels

Anti-HBs is the antibody produced in response to hepatitis B surface antigen (HBsAg), either through natural infection or vaccination. The interpretation depends on the clinical context:

In Previously Vaccinated Individuals:

• Anti-HBs < 10 mIU/mL: Considered non-protective 1
• Anti-HBs ≥ 10 mIU/mL: Considered protective against both acute disease and chronic infection 1

Therefore, an anti-HBs level < 5 mIU/mL falls below the protective threshold and indicates:

• Inadequate response to previous vaccination
• Waning antibody levels after previous vaccination
• Lack of previous vaccination or natural infection

Clinical Implications

For Healthcare Personnel (HCP):

1. If previously vaccinated but anti-HBs < 10 mIU/mL:
   • Should receive one dose of HepB vaccine
   • Retest anti-HBs 1-2 months later
   • If still < 10 mIU/mL, complete a second vaccine series (2 more doses)
   • Retest after completion 1

For General Population:

1. If never vaccinated:

   • Complete HepB vaccine series (2 or 3 doses depending on vaccine)
   • No post-vaccination testing generally needed for most adults 2

2. If previously vaccinated:

   • For most healthy adults, no booster needed even if anti-HBs < 10 mIU/mL
   • For high-risk individuals (healthcare workers, immunocompromised patients), follow testing and revaccination protocols 1

Important Considerations

Immune Memory

Despite low antibody levels:

• Many individuals maintain immune memory that can provide protection
• Studies show that vaccinated individuals with anti-HBs < 10 mIU/mL may still have protective immunity through memory B and T cells 3, 4
• Research indicates that anti-HBs levels ≥ 2 IU/L may predict an anamnestic response to a booster dose 5

Special Populations

For patients with inflammatory bowel disease or immunosuppression:

• Those with anti-HBs < 10 mIU/mL should receive a challenge dose
• If no response, complete a full vaccination series 1

Sex Differences

• Females generally show stronger humoral immune responses to hepatitis B vaccination
• Females demonstrate better response to booster doses than males 6

Management Algorithm

1. Check vaccination history
2. Determine risk category (healthcare worker, immunocompromised, general population)
3. If anti-HBs < 5 mIU/mL:
   • For healthcare workers: Administer one dose of HepB vaccine, check anti-HBs in 1-2 months
   • For immunocompromised: Administer one challenge dose, check response in 4-8 weeks
   • For general population with completed vaccination: No action needed unless high-risk exposure
   • For unvaccinated individuals: Complete full vaccination series

Conclusion on Protection

While 10 mIU/mL is the established threshold for protection, research suggests:

• No absolute level guarantees complete protection against infection
• Anti-HBs ≥ 10 mIU/mL protects against chronic HBV carriage 7
• Even with anti-HBs < 10 mIU/mL, many individuals maintain immune memory that may provide protection 4