What is the false‑positive rate of hepatitis A IgM serology (anti‑HAV assay)?

False-Positive Rate of Hepatitis A IgM Testing

False-positive HAV IgM results occur frequently in low-prevalence populations, with approximately 35% (range: 15-60%) of positive screening tests being false-positives when testing populations with <10% prevalence of acute hepatitis A 1.

Understanding the Problem

The false-positive rate of hepatitis A IgM testing is critically dependent on the prevalence of disease in the population being tested. This is a fundamental issue of positive predictive value that clinicians must understand to avoid misdiagnosis.

Key Evidence from Guidelines:

• In low-prevalence populations (<10% prevalence, such as general population screening, healthcare workers, or STD clinic attendees): The false-positive rate averages approximately 35% (range: 15-60%) 1

• The 2018 IDSA/ASM guidelines explicitly warn that false-positive HAV IgM results occur due to low positive predictive value of assays when used in populations with low prevalence of acute hepatitis A 1

• The 2006 ACIP guidelines confirm that likely false-positive tests occur in persons without evidence of recent HAV infection 2

Clinical Context Matters

When False-Positives Are Most Problematic:

• Asymptomatic screening in low-prevalence areas
• Testing without clinical context or liver enzyme elevation
• Patients without jaundice or typical hepatitis symptoms
• Testing >1 year after potential exposure (persistent IgM can occur but is often false-positive)

When Results Are More Reliable:

Recent research 3 demonstrates that all confirmed acute HAV cases had anti-HAV IgM values >4.0 on the Architect platform, with mean values of 9.4. In contrast, low-level reactive results (closer to the cutoff) were associated with alternative diagnoses in 63.6% of cases where clinical data was available.

Critical Pitfalls to Avoid:

1. Never diagnose acute hepatitis A based on IgM alone without clinical correlation 1

2. Always correlate with:

   • Elevated liver enzymes (mean peak ALT ~1920 in confirmed cases) 3
   • Clinical jaundice (present in 90% of confirmed adult cases) 3
   • Epidemiologic risk factors
   • Signal-to-cutoff ratio (higher ratios more reliable)

3. Be aware of assay-specific issues:

   • Different platforms have varying sensitivity for IgM detection 4
   • Some automated analyzers can cause HAV RNA contamination during reflex testing 5
   • Rheumatoid factor-like substances can cause false-positives 6

4. Persistent positive IgM >1 year after infection should raise suspicion for false-positivity 2

Recommended Diagnostic Approach:

For suspected acute hepatitis A:

• Order HAV IgM only in patients with clinical hepatitis (elevated ALT, jaundice) or clear epidemiologic exposure
• If IgM positive with low signal-to-cutoff ratio and atypical presentation, consider:
  • Repeat testing in 7-10 days
  • HAV RNA testing (if available) for confirmation
  • Alternative diagnoses (autoimmune hepatitis has been misdiagnosed as HAV) 7

For screening asymptomatic individuals:

• Use total anti-HAV (IgM + IgG) to assess immunity status, not IgM alone 2
• Positive total anti-HAV indicates past infection or vaccination
• IgM testing in asymptomatic persons has unacceptably high false-positive rates

The low positive predictive value in low-prevalence settings makes clinical correlation absolutely essential before acting on positive HAV IgM results, particularly for infection control decisions like patient isolation or public health reporting.