A 48-year-old man from Mumbai with a history of intravenous drug use presents with fatigue, jaundice, and arthralgia for 4 weeks. He denies recent water exposure or shellfish consumption. Laboratory investigations show: ALT 1600 IU/L, AST 1400 IU/L, alkaline phosphatase 220 IU/L, total bilirubin 7.5 mg/dL, INR 1.8, platelet count 120,000/μL. Serological tests reveal: HBsAg positive, anti-HBc IgM positive, anti-HAV IgG positive, anti-HEV IgG negative. Abdominal ultrasound shows hepatomegaly with normal portal vein flow. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Hepatitis B Superinfection (Reactivation) ### Serological Interpretation | Marker | Result | Interpretation | |--------|--------|----------------| | HBsAg | Positive | Indicates HBV infection (acute or chronic) | | Anti-HBc IgM | Positive | **Acute viral replication** — current/recent infection | | Anti-HAV IgG | Positive | Prior HAV exposure/immunity; not acute HAV | | Anti-HEV IgG | Negative | No prior HEV exposure | **Key Point:** The combination of **HBsAg + anti-HBc IgM** defines acute hepatitis B. The presence of anti-HAV IgG (not IgM) rules out acute HAV superinfection. ### Clinical Context: IVDU & Chronic HBV Carrier State This patient's risk profile (intravenous drug use) and serological pattern suggest he is a **chronic HBsAg carrier** who has now developed **acute reactivation** or **acute superinfection** with HBV. The anti-HBc IgM positivity indicates recent viral replication flare. **High-Yield:** In endemic regions and among IVDU populations, many individuals are chronic HBsAg carriers with low-level viral replication. Acute flares can occur spontaneously or after exposure to a new HBV strain. ### Biochemical & Coagulation Abnormalities - **Elevated transaminases**: ALT 1600, AST 1400 — consistent with acute hepatocellular injury. - **INR 1.8**: indicates **impaired synthetic function** — more severe than simple acute HAV (INR typically < 1.3 in uncomplicated acute hepatitis). - **Thrombocytopenia (120,000/μL)**: suggests portal hypertension or bone marrow suppression from severe hepatitis. - **Cholestasis component** (ALP 220): mixed hepatocellular-cholestatic pattern. ### Pathological Features of Acute HBV Reactivation - **Acute inflammation**: portal and lobular infiltration; hepatocyte ballooning. - **Bridging necrosis**: may progress to submassive necrosis in severe cases. - **Cholestasis**: bile plugs; hepatocellular dysfunction. - **Risk of progression**: INR > 1.5 and thrombocytopenia indicate potential for acute liver failure in a subset of patients [cite:Robbins 10e Ch 20]. ### Natural History Unlike acute HAV (which is self-limited), acute HBV reactivation in a chronic carrier can result in: - **Complete clearance** (seroconversion to anti-HBs) — ~5–10% of chronic carriers. - **Continued chronic infection** — majority remain HBsAg+ with fluctuating ALT. - **Fulminant failure** — rare but possible if INR worsens and encephalopathy develops. **Clinical Pearl:** The presence of anti-HAV IgG (not IgM) indicates this patient has prior immunity to HAV and is not experiencing acute HAV superinfection — a common co-infection in IVDU but ruled out here by negative anti-HAV IgM.