Testing for Current Hepatitis A Infection
Order IgM anti-HAV (immunoglobulin M antibody to hepatitis A virus) to diagnose acute hepatitis A infection. 1
Primary Diagnostic Test
- IgM anti-HAV is the first-line test for confirming acute hepatitis A in patients presenting with symptoms of acute hepatitis 1
- This test should be performed on serum or plasma using enzyme immunoassay 1
- IgM anti-HAV becomes detectable 5-10 days before symptom onset and typically remains positive for up to 6 months after infection 1
Critical Timing Considerations and Window Period
Be aware that approximately 11% of acute hepatitis A patients will have a false-negative IgM anti-HAV result at initial presentation due to testing during the window period. 2
- Patients tested very early in their illness (shorter time from symptom onset) are more likely to have initially negative serology 2
- If clinical suspicion is high but initial IgM anti-HAV is negative, repeat testing in 3-7 days 2, 3
- Patients with fever, lower bilirubin levels, and higher cutoff index (COI) values are most likely to seroconvert on repeat testing 2
Alternative Testing Strategy
- Total anti-HAV testing alone is NOT adequate for diagnosing acute infection because it cannot differentiate current infection from past infection or vaccination-induced immunity 1, 4
- However, some total anti-HAV assays have higher sensitivity for detecting IgM antibodies than dedicated IgM assays, which can cause diagnostic confusion 3
- In rare early acute cases, anti-HAV total may be reactive while IgM anti-HAV is non-reactive due to assay sensitivity differences 3
Specimen Collection Requirements
- Collect blood in serum separator tubes or EDTA-treated plasma tubes 1
- Specimens must be kept at ambient temperature and delivered to the laboratory within 2 hours to preserve antibody integrity 1
Advanced Testing for Equivocal Cases
- HAV RNA testing by nucleic acid amplification (NAAT) can be performed on plasma or serum when serology is equivocal or when very early infection is suspected 1
- This molecular testing is particularly useful during the serologic window period 1
Common Pitfalls to Avoid
- Do not confuse hepatitis A antibody patterns with hepatitis B antibody patterns, which have completely different interpretations 1
- Do not rely on a single negative IgM anti-HAV result to exclude hepatitis A in patients with early symptoms of acute hepatitis 2, 3
- Do not order only total anti-HAV for acute diagnosis, as this will miss the distinction between acute and past infection 1, 4