## Definition of MDR-TB **Key Point:** MDR-TB (Multidrug-resistant TB) is defined as resistance to at least the two most potent first-line anti-TB drugs: **isoniazid (INH)** and **rifampicin (RIF)**. ### Why These Two Drugs? Isoniazid and rifampicin are the backbone of standard TB treatment: - **Rifampicin** is the most potent anti-TB agent with the fastest bactericidal activity - **Isoniazid** is highly effective against actively dividing bacilli and penetrates well into tissues - Together, they account for the majority of treatment success in drug-susceptible TB ### Clinical Significance **High-Yield:** Resistance to these two drugs alone (without XDR features) defines MDR-TB and requires prolonged treatment with second-line drugs (fluoroquinolones, injectable agents, bedaquiline, linezolid). **Clinical Pearl:** A patient with TB resistant only to isoniazid (mono-resistant) or only to rifampicin (mono-resistant) is NOT classified as MDR-TB and may still respond to standard regimens with modifications. ### WHO Classification Hierarchy | Resistance Pattern | Definition | | --- | --- | | **MDR-TB** | Resistant to INH + RIF (± other drugs) | | **XDR-TB** | MDR-TB + resistant to any fluoroquinolone + any injectable (amikacin/capreomycin/kanamycin) | | **RR-TB** | Resistant to RIF alone (often associated with INH resistance) | [cite:WHO TB Guidelines 2023]
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