A 28-year-old woman with known chronic hepatitis B (HBsAg positive for 5 years) presents with acute worsening of jaundice, INR 2.8, and ALT 4200 IU/L. Testing shows HBeAg positive, HBV DNA 8 × 10^6 IU/mL, and anti-HDV IgM positive. Which single feature best distinguishes acute hepatitis D superinfection from acute exacerbation of chronic hepatitis B?

Explanation

## Distinguishing Acute Hepatitis D Superinfection from Acute Exacerbation of Chronic Hepatitis B ### Serological Diagnosis: Anti-HDV IgM **Key Point:** Anti-HDV IgM is the definitive serological marker of acute hepatitis D infection (superinfection in a chronic HBsAg carrier) and is the single feature that reliably distinguishes it from acute exacerbation of chronic HBV alone. ### Comparison Table: Acute Exacerbation HBV vs Acute HDV Superinfection | Feature | Acute HBV Exacerbation | Acute HDV Superinfection | |---------|----------------------|-------------------------| | **Anti-HDV serology** | Negative | Anti-HDV IgM positive | | **HBsAg status** | Positive (chronic carrier) | Positive (chronic carrier) | | **ALT elevation** | Often >1000 IU/L | Often >2000 IU/L (more severe) | | **Fulminant risk** | 0.5–1% | 5–15% (much higher) | | **Prognosis** | Exacerbation resolves or progresses to cirrhosis | High risk of acute liver failure | | **HDV RNA** | Absent | Present (HDV RNA detectable) | ### Why Anti-HDV IgM is the Discriminator 1. **Specific for acute HDV infection:** Anti-HDV IgM appears only during acute hepatitis D and is absent in chronic HDV or in HBV exacerbation without HDV. 2. **Diagnostic gold standard:** While both conditions present with acute decompensation in a chronic HBsAg carrier, only HDV superinfection triggers anti-HDV IgM production. 3. **Clinical significance:** HDV superinfection carries a much higher risk of fulminant hepatic failure (5–15%) compared to HBV exacerbation alone (0.5–1%), making serological confirmation critical for prognostication and management. **Clinical Pearl:** Hepatitis D (delta) is a defective RNA virus that requires HBsAg for replication. It can only infect HBsAg-positive individuals. Acute superinfection in a chronic HBsAg carrier presents with severe acute hepatitis and a high risk of fulminant failure. **Mnemonic:** **HDV = Defective virus (needs HBV helper).** Always check anti-HDV in any HBsAg-positive patient with acute hepatitis or acute exacerbation. **High-Yield:** Anti-HDV IgM is the acute marker; anti-HDV IgG indicates past or chronic infection. HDV RNA PCR is the most sensitive test for active replication. [cite:Harrison 21e Ch 304]