## Definition of XDR-TB **Key Point:** XDR-TB = MDR-TB (INH + RIF resistance) + resistance to ≥1 fluoroquinolone AND ≥1 injectable agent (amikacin, capreomycin, or streptomycin). **High-Yield:** The two critical second-line drug classes that define XDR-TB are: 1. **Fluoroquinolones** (levofloxacin, moxifloxacin) — oral agents 2. **Injectable agents** (amikacin, capreomycin, streptomycin) — parenteral agents Both resistances must be present for XDR-TB classification. ## XDR-TB vs MDR-TB Comparison | Feature | MDR-TB | XDR-TB | |---------|--------|--------| | **Core resistance** | INH + RIF | INH + RIF | | **Additional resistance** | None required | ≥1 FQ + ≥1 injectable | | **Treatment duration** | 20 months | 20+ months (often 24 months) | | **Cure rate** | ~55–60% | ~40–50% | | **Mortality risk** | Moderate | High | **Clinical Pearl:** XDR-TB is a public health emergency. The loss of fluoroquinolones (the most potent oral second-line agents) and injectables forces reliance on newer agents like bedaquiline, linezolid, and delamanid — drugs that are expensive, require longer therapy, and have limited global availability. **Mnemonic:** **XDR = MDR + FQ + Injectable** — think of it as "MDR on steroids" with two additional critical drug class resistances. [cite:WHO TB Guidelines 2023, Park 26e Ch 8]
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