What does a positive IgM (Immunoglobulin M) versus IgG (Immunoglobulin G) test result indicate for Hepatitis A (Hep A) infection?

Interpreting Hepatitis A Antibody Test Results

A positive IgM antibody test for Hepatitis A indicates an acute or recent infection, while a positive IgG antibody test indicates past infection or immunity from either previous infection or vaccination. 1

Detailed Interpretation of Hepatitis A Antibody Tests

IgM Antibody (Anti-HAV IgM)

• Positive result: Indicates acute or recent HAV infection
• Appears early in infection, typically detectable 4-6 weeks after exposure
• Usually becomes positive before symptom onset
• Remains detectable for 2-4 months in most cases, but can persist for variable periods:
  • Some patients become negative within 30 days
  • Most patients become negative by 120 days
  • Some patients (13.5%) may remain positive for >200 days 2
  • Rarely can persist up to 420 days 2

IgG Antibody (Anti-HAV IgG)

• Positive result: Indicates either:
  • Past/resolved hepatitis A infection
  • Immunity due to vaccination
• Appears later in the course of infection
• Persists for life, providing long-term immunity

Clinical Interpretation Algorithm

1. Positive IgM, Negative IgG:

   • Early acute HAV infection
   • Patient is likely infectious
   • Monitor for liver function abnormalities

2. Positive IgM, Positive IgG:

   • Recent HAV infection in recovery phase
   • Patient may still be infectious

3. Negative IgM, Positive IgG:

   • Past HAV infection with immunity
   • Or successful vaccination
   • Patient is not infectious

4. Negative IgM, Negative IgG:

   • No current or past HAV infection
   • No immunity (susceptible to infection)

Important Considerations and Pitfalls

False Positive IgM Results

• Low positive predictive value in populations with low prevalence of acute hepatitis A 1
• Can occur due to:
  • Polyclonal immune stimulation
  • Immune reactivation 3
  • Other liver diseases 4

Testing Timing Considerations

• False negative IgM results may occur if testing is done too early
  • Some patients test negative within the first 3 days after symptom onset 5
• IgM levels typically peak 2-4 weeks after symptom onset 5

Improving Diagnostic Accuracy

• Consider clinical presentation alongside test results
• For uncertain cases, IgG avidity testing can help differentiate:
  • Acute infection: avidity <50% in first month
  • Past infection: avidity >70% 3
• Repeat testing in 1-2 weeks if clinical suspicion is high but initial IgM is negative

Monitoring and Management Implications

• Positive IgM indicates need for:
  • Liver function monitoring
  • Precautions to prevent transmission
  • Notification of public health authorities
• Consider testing for other viral hepatitis (B, C, E) in patients with liver abnormalities
• Patients with resolved HAV infection (IgG positive) have lifelong immunity and do not require vaccination

By understanding the pattern of antibody results, clinicians can accurately diagnose acute hepatitis A infection, past infection, or immunity status, and provide appropriate management.