## Definition of MDR-TB **Key Point:** MDR-TB (Multidrug-resistant TB) is defined as TB caused by *Mycobacterium tuberculosis* strains resistant to at least **isoniazid (INH) and rifampicin (RIF)** — the two most potent first-line agents. ### Why These Two Drugs? 1. **Isoniazid and rifampicin** are the backbone of standard TB therapy 2. Resistance to both confers high treatment failure risk 3. Requires prolonged second-line drug regimens (18–20 months) ### Classification Hierarchy | Resistance Pattern | Definition | Treatment Duration | |---|---|---| | **MDR-TB** | INH + RIF | 18–20 months (second-line) | | **XDR-TB** | MDR + FQ + SLI | 20–24 months (third-line) | | **Pre-XDR-TB** | MDR + FQ OR SLI (not both) | 18–20 months | **High-Yield:** The WHO and Indian TB guidelines (NTEP) define MDR-TB strictly as INH + RIF resistance. Any other combination (e.g., INH + ethambutol, or RIF + streptomycin alone) is NOT classified as MDR-TB. **Clinical Pearl:** A patient with INH resistance alone is treated with standard first-line drugs but with extended duration. Only when RIF is also resistant does the regimen shift to second-line agents.
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