## Distinguishing Feature: Treatment Duration and Backbone Drugs ### Drug-Susceptible TB (DS-TB) **Key Point:** Standard regimen is 2-month intensive phase (HRZE) followed by 4-month continuation phase (HR), totaling **6 months**. - Backbone drugs: Isoniazid (H), Rifampicin (R), Pyrazinamide (Z), Ethambutol (E) - Intensive phase: 2 months HRZE - Continuation phase: 4 months HR - Success rate: 85–90% with adherence ### Multidrug-Resistant TB (MDR-TB) **Key Point:** Resistant to both isoniazid AND rifampicin. Requires **20 months or longer** with second-line drugs. - Intensive phase: 8 months with fluoroquinolone (levofloxacin/moxifloxacin) + injectable (amikacin/streptomycin) + pyrazinamide ± linezolid - Continuation phase: 12 months with fluoroquinolone + pyrazinamide ± linezolid - Total: 20 months minimum (WHO 2019 shorter regimen); traditional: 24 months - Success rate: 55–65% depending on regimen and adherence ### Comparison Table | Feature | DS-TB | MDR-TB | | --- | --- | --- | | **Total duration** | 6 months | 20+ months | | **Intensive phase** | 2 months | 8 months | | **Continuation phase** | 4 months | 12 months | | **First-line drugs** | H, R, Z, E | Not effective | | **Second-line drugs** | Not used | Fluoroquinolone, injectable, linezolid | | **Success rate** | 85–90% | 55–65% | **High-Yield:** The **20-month minimum duration** is the single best discriminator between DS-TB and MDR-TB. This is a high-frequency NEET PG testing point because it reflects the dramatic difference in complexity and duration of therapy. **Clinical Pearl:** Even with newer shorter MDR-TB regimens (20 months), the duration is still 3.3× longer than DS-TB. Adherence and toxicity monitoring become critical challenges in MDR-TB management. **Mnemonic:** **"DS = 6, MDR = 20"** — a quick recall aid for the duration ratio.
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