A 32-year-old man with newly diagnosed pulmonary tuberculosis is started on standard four-drug regimen (HRZE). After 2 months of therapy, he develops sudden-onset jaundice, dark urine, and right upper quadrant pain. Liver function tests show ALT 320 U/L, AST 380 U/L, and total bilirubin 4.2 mg/dL. Which investigation is most appropriate to determine the causative antitubercular drug responsible for hepatotoxicity?

Explanation

## Investigation of Choice for Drug-Induced Hepatotoxicity in TB Therapy ### Clinical Context Drug-induced liver injury (DILI) during antitubercular therapy is a common and serious adverse effect. Among HRZE, isoniazid and rifampicin are the most hepatotoxic agents, followed by pyrazinamide. ### Why Rechallenge Test is Gold Standard **Key Point:** The rechallenge test (also called re-introduction test or drug challenge) is the gold standard for identifying which specific antitubercular drug is causing hepatotoxicity. **Clinical Pearl:** The protocol involves: 1. Stopping all four drugs immediately 2. Waiting for LFTs to normalize (typically 3–7 days) 3. Reintroducing each drug sequentially at 3–5 day intervals 4. Monitoring LFTs after each reintroduction 5. The drug causing LFT deterioration is the culprit ### Hierarchy of Hepatotoxicity Risk | Drug | Hepatotoxicity Incidence | Mechanism | Severity | |------|--------------------------|-----------|----------| | Isoniazid | 0.5–3% | Acetylator polymorphism; toxic metabolite | Moderate to severe | | Rifampicin | 0.8–2% | Enzyme induction; direct hepatotoxicity | Moderate | | Pyrazinamide | 1–5% | Hyperuricemia + direct injury | Moderate | | Ethambutol | <0.1% | Rare; usually with renal impairment | Mild | **High-Yield:** Slow acetylators of isoniazid have higher risk of hepatotoxicity due to accumulation of toxic metabolites (acetylhydrazine). ### Why Rechallenge Test is Superior - **Specificity:** Directly identifies the offending agent - **Reproducibility:** Positive rechallenge confirms causality (Naranjo score) - **Guides future therapy:** Allows continuation of safe drugs and substitution of the culprit - **Cost-effective:** No need for expensive serologies or imaging **Tip:** Rechallenge should be done in a controlled setting with close LFT monitoring. If the patient is severely ill, defer rechallenge until clinically stable.