## Definition of MDR-TB **Key Point:** MDR-TB is defined as tuberculosis caused by *Mycobacterium tuberculosis* strains resistant to at least isoniazid (INH) and rifampicin (RIF) — the two most potent first-line agents. **High-Yield:** This is the WHO and Indian TB guidelines definition. MDR-TB does NOT require resistance to fluoroquinolones or other second-line drugs; those additional resistances define XDR-TB. ## Classification Hierarchy | TB Type | Definition | |---------|------------| | **Drug-susceptible TB** | Susceptible to INH and RIF | | **MDR-TB** | Resistant to INH + RIF (± other first-line drugs) | | **XDR-TB** | MDR-TB + resistance to ≥1 fluoroquinolone + ≥1 injectable (amikacin, capreomycin, or linezolid) | | **RR-TB** | Resistant to RIF alone (includes MDR-TB) | **Clinical Pearl:** The combination of INH and RIF resistance is critical because these two drugs are synergistic and form the backbone of standard TB therapy. Loss of both dramatically reduces treatment efficacy and prolongs therapy to 20 months or more. **Mnemonic:** **MDR = INH + RIF** — remember the two most important drugs are the defining pair for MDR. [cite:WHO TB Guidelines 2023, Park 26e Ch 8]
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