A 35-year-old man from Mumbai presents with jaundice and malaise for 2 weeks. Liver function tests show elevated transaminases (ALT 1200 U/L). HBsAg is positive. Which single investigation is most appropriate to determine if this is acute HBV infection versus chronic HBV with acute exacerbation?

Explanation

## Interpretation of HBV Serology in Acute vs Chronic Infection **Key Point:** Anti-HBc IgM is the gold standard marker for acute HBV infection and appears early in the window period when HBsAg is positive but anti-HBs is still negative. ### Serological Markers Timeline | Marker | Acute HBV | Chronic HBV | Window Period | Immunity (Post-vaccination) | |--------|-----------|-------------|---------------|---------------------------| | **HBsAg** | Positive (early) | Positive (persistent >6 mo) | Negative | Negative | | **Anti-HBc IgM** | **Positive** | Negative | **Positive** | Negative | | **Anti-HBc IgG** | Appears late | Positive | Positive | Negative | | **Anti-HBs** | Appears during recovery | Negative | Negative | **Positive** | | **HBeAg** | Often positive | Variable | Variable | Negative | **High-Yield:** Anti-HBc IgM is the **only marker that definitively distinguishes acute from chronic HBV infection** when HBsAg is positive. ### Clinical Application 1. **Acute HBV infection:** HBsAg(+) + Anti-HBc IgM(+) = Recent/acute infection 2. **Chronic HBV infection:** HBsAg(+) + Anti-HBc IgM(−) = Chronic carrier or chronic active hepatitis 3. **Window period:** HBsAg(−) + Anti-HBc(+) + Anti-HBc IgM(+) = Early acute infection **Clinical Pearl:** In this patient with acute presentation (2-week jaundice, markedly elevated ALT), anti-HBc IgM positivity would confirm acute HBV infection, whereas its absence would suggest chronic HBV with acute flare or superinfection with another hepatotropic virus. **Mnemonic:** **IgM = Immediate/Infection** — IgM antibodies appear early in acute infection; IgG appears later and persists in chronic infection.