Laboratory Diagnosis of Hepatitis A
The primary laboratory test for diagnosing acute hepatitis A infection is serum or plasma IgM anti-HAV antibody, which should be detected using enzyme immunoassay. 1
Optimal Specimen Collection
- Collect serum or plasma for IgM anti-HAV testing 1
- Use EDTA or plasma preparation tube (PPT) for plasma samples, or clot/serum separator tube (SST) for serum 1
- Transport at room temperature within 2 hours of collection 1
Understanding the Diagnostic Window
IgM anti-HAV becomes detectable 5-10 days before symptom onset and typically persists for up to 6 months after acute infection. 2 However, critical diagnostic pitfalls exist:
- 10.9% of acute hepatitis A patients test negative on initial IgM anti-HAV testing due to testing during the window period 3
- Patients with negative initial serology typically present earlier in their illness, have higher fever rates, and lower ALT/bilirubin levels 3
- IgM anti-HAV can persist beyond 6 months in 13.5% of patients, with some remaining positive for over 200 days 4
When to Repeat Testing
If initial IgM anti-HAV is negative but acute hepatitis A is clinically suspected, repeat testing at least 2 days after peak ALT levels. 5 Key indicators for repeat testing include:
- Shorter interval from symptom onset to initial test 3
- Presence of fever 3
- Lower bilirubin levels 3
- Higher cutoff index (COI) values on the initial test 3
Distinguishing Acute from Past Infection
- Total anti-HAV (combined IgM and IgG) indicates either current infection, past infection, or vaccination-induced immunity but cannot differentiate between these states 1, 2
- IgM anti-HAV positivity specifically indicates acute or recent infection (within past 6 months) 2
- IgM anti-HAV negativity with total anti-HAV positivity indicates past infection or vaccination 2
Advanced Diagnostic Considerations
For complex cases where serology is equivocal:
- HAV RNA quantitative testing by nucleic acid amplification (NAAT) can be performed on plasma or serum 1
- IgG avidity testing can distinguish acute infection (low avidity, <50%) from past infection or immune reactivation (high avidity, >70%), particularly useful in elderly patients 6
- HAV biomarker 2A can differentiate previously infected from vaccinated individuals with 92% accuracy, though this is primarily a research tool 7
Clinical Context
All patients with biochemical evidence of acute hepatitis should undergo IgM anti-HAV testing as part of first-line virological investigation. 8 This is particularly important because: