## Distinguishing Acute Hepatitis B from Acute Hepatitis A ### Key Serological Markers **Key Point:** Anti-HBc IgM is the most reliable early marker of acute hepatitis B infection and appears during the window period when HBsAg is positive but anti-HBs has not yet developed. ### Comparative Table: Hepatitis A vs Hepatitis B | Feature | Hepatitis A | Hepatitis B | | --- | --- | --- | | **Transmission** | Fecal-oral (contaminated water/food) | Parenteral, sexual, vertical | | **Acute fulminance** | <1% (except in elderly/cirrhotic) | 0.1–0.5% in immunocompetent | | **Chronicity** | Never (100% resolution) | 5–10% in adults, 90% in neonates | | **Early marker (acute phase)** | Anti-HAV IgM | Anti-HBc IgM | | **Window period marker** | None (anti-HAV IgM bridges to anti-HAV IgG) | Anti-HBc IgM (HBsAg negative, anti-HBs negative) | | **Histology** | Acute inflammation, no bridging necrosis | Acute inflammation, may have bridging necrosis | ### Clinical Pearl **High-Yield:** Anti-HBc IgM is the **gold standard** for diagnosing acute HBV infection, especially in the window period (first 4–6 weeks) when HBsAg may be transiently negative but the patient is still infectious. This is the single best discriminator between acute HAV and acute HBV. ### Why Other Features Do Not Distinguish - **Fulminant failure:** Rare in both; HAV can cause fulminance in elderly/cirrhotic patients, HBV rarely in immunocompetent adults. Not a reliable discriminator. - **Fecal-oral transmission:** This is HAV's hallmark, but the question asks what **distinguishes** acute HBV from HAV — this feature is present in HAV, not HBV, so it does not help identify which virus is causing the acute illness in a given patient. - **Complete resolution:** Both HAV and acute HBV resolve completely in the acute phase in immunocompetent hosts; chronicity develops later in HBV. Not a feature of acute illness.
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