A 35-year-old man with newly diagnosed pulmonary TB is started on standard first-line therapy. After 3 weeks, he develops jaundice, malaise, and a rash. LFTs show AST 180 U/L, ALT 160 U/L, bilirubin 3.2 mg/dL. Which single feature best distinguishes drug-induced liver injury from TB-related hepatitis in this scenario?

Explanation

## Distinguishing Drug-Induced Liver Injury from TB Hepatitis ### Clinical Context In a TB patient on first-line therapy presenting with jaundice and elevated transaminases, the key question is: **Is this drug hepatotoxicity (isoniazid, rifampicin, or pyrazinamide) or TB-related hepatitis (granulomatous disease)?** ### Discriminating Features | Feature | Drug-Induced Liver Injury (DILI) | TB Hepatitis (Granulomatous) | |---------|-----------------------------------|------------------------------| | **Temporal onset** | 2–8 weeks after drug initiation | Variable; may precede or follow TB diagnosis | | **Resolution pattern** | Rapid improvement (days–weeks) after drug withdrawal | Slower resolution; may persist despite TB treatment | | **Causative agent** | Rifampicin (most common), INH, PZA | *Mycobacterium tuberculosis* granulomas | | **Histology** | Hepatocellular necrosis, cholestasis; no organisms | Caseating granulomas; AFB may be visible | | **Response to rechallenge** | Recurrence of LFT elevation with drug restart | No change with TB drug restart | | **ALP elevation** | Mild to moderate; proportionate to transaminases | Often marked ALP elevation (cholestasis pattern) | | **Associated features** | Rash, fever, eosinophilia (hypersensitivity pattern) | Constitutional TB symptoms (fever, weight loss, cough) | ### Key Point **Key Point:** The **temporal relationship to drug initiation and rapid resolution upon drug withdrawal** is the gold standard discriminator. Drug-induced hepatotoxicity resolves within days to weeks of stopping the offending agent, whereas TB hepatitis (granulomatous) resolves more slowly and does not show this immediate temporal link. ### Clinical Pearl **Clinical Pearl:** In practice, when a TB patient develops jaundice 2–8 weeks into therapy: 1. **Stop all TB drugs immediately** (or switch to non-hepatotoxic regimen) 2. **Monitor LFTs daily** — if they improve rapidly (within 3–5 days), it was DILI 3. **If LFTs plateau or worsen**, suspect TB hepatitis or another cause (viral hepatitis, alcohol) 4. **Rechallenge cautiously** after LFTs normalize — DILI will recur; TB hepatitis will not ### High-Yield Mnemonic **Mnemonic:** **RAPID** (Rapid resolution After drug Pullback = drug-Induced; Persistent = TB-related) ### Tip **Tip:** Exam questions often test the **rechallenge phenomenon**: if LFTs worsen again when drugs are reintroduced, it confirms DILI. This is the most specific discriminator, but temporal relationship + rapid resolution is the quickest clinical discriminator.