## Definition and Classification of Drug-Resistant TB **Key Point:** MDR-TB and XDR-TB are distinct entities defined by their resistance profiles, not by clinical presentation alone. ### MDR-TB (Multidrug-Resistant TB) - Resistance to **at least isoniazid AND rifampicin** (the two most potent first-line drugs) - May or may not be resistant to other first-line agents (ethambutol, pyrazinamide) - Represents failure of standard 6-month regimen ### XDR-TB (Extensively Drug-Resistant TB) - MDR-TB **PLUS** resistance to **at least one fluoroquinolone** (levofloxacin or moxifloxacin) - **AND** resistance to **at least one injectable second-line drug** (amikacin, capreomycin, or streptomycin) - Subset of MDR-TB with worse prognosis ### Comparison Table | Feature | MDR-TB | XDR-TB | |---------|--------|--------| | **Isoniazid resistance** | Yes (required) | Yes (required) | | **Rifampicin resistance** | Yes (required) | Yes (required) | | **Fluoroquinolone resistance** | No (not required) | Yes (required) | | **Injectable SLD resistance** | No (not required) | Yes (required) | | **Treatment duration** | 20 months (WHO 2019) | 20+ months; individualized | | **Cure rate** | ~60–70% | ~40–50% | **High-Yield:** The defining discriminator between MDR and XDR is the addition of fluoroquinolone AND injectable drug resistance to the MDR backbone. This is the WHO/CDC standard definition used globally and in NEET PG. **Clinical Pearl:** XDR-TB has significantly worse outcomes and requires newer agents (bedaquiline, linezolid, delamanid) for adequate treatment. MDR-TB can still be cured with optimized second-line regimens. [cite:Harrison 21e Ch 158]
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