Multidrug-resistant tuberculosis - definition and treatment implications
Core concept
| Term | Definition |
|---|---|
| Mono-resistant | Resistance to one first-line drug |
| RR-TB | Rifampicin-resistant, regardless of isoniazid - treated as MDR because rifampicin is the backbone drug |
| MDR-TB | Resistant to isoniazid AND rifampicin |
| Pre-XDR-TB | MDR/RR-TB PLUS resistance to any fluoroquinolone |
| XDR-TB (WHO 2021 revision) | MDR/RR + fluoroquinolone + at least one Group A drug (bedaquiline or linezolid) |
- The 2021 redefinition dropped "injectable" from the XDR definition because injectables are no longer core therapy - an old answer will describe resistance to a second-line injectable, which is out of date
- Resistance mechanisms are chromosomal point mutations, not plasmids
- Rifampicin - rpoB (>95%, the target of Xpert MTB/RIF)
- Isoniazid - katG (high-level) and inhA promoter (low-level, cross-resistance with ethionamide)
- Fluoroquinolone - gyrA; bedaquiline - atpE/Rv0678; pyrazinamide - pncA
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