Definitions and Resistance Patterns of Drug‑Resistant Tuberculosis
1. Core Definitions
- Multidrug‑resistant TB (MDR‑TB) is defined as Mycobacterium tuberculosis that is resistant to at least rifampin and isoniazid. 1
- Pre‑extensively drug‑resistant TB (Pre‑XDR‑TB) refers to MDR‑TB with resistance to either a fluoroquinolone or an injectable second‑line drug (but not both); this category guides treatment planning. [1] [3]
- Extensively drug‑resistant TB (XDR‑TB) – 2020 definition: MDR‑TB plus resistance to any fluoroquinolone and to bedaquiline and/or linezolid (reflecting the prognostic importance of these newer agents). 2
- XDR‑TB – pre‑2020 definition: MDR‑TB plus resistance to any fluoroquinolone and to any second‑line injectable drug. 2
2. MDR‑TB Treatment Regimen Structure
- Intensive phase should contain ≥ 5 effective drugs; the continuation phase should contain ≥ 4 effective drugs. (Strong recommendation) 3
- The intensive phase lasts 5–7 months after sputum culture conversion. (Strong recommendation) 3
- Total treatment duration for MDR‑TB is 15–21 months after culture conversion. (Strong recommendation) 5
3. Preferred (Group A) Drugs for MDR‑TB
| Drug (Group A) | Recommendation Strength | Key Points |
|---|---|---|
| Later‑generation fluoroquinolone (levofloxacin or moxifloxacin) | Strong | Central to regimen; high efficacy. [3] |
| Bedaquiline | Strong | Cornerstone of modern MDR‑TB therapy. [5] |
4. Conditionally Recommended (Group B/C) Drugs for MDR‑TB
- Linezolid – conditional recommendation; dose‑adjusted regimens (e.g., 600 mg) reduce adverse events while maintaining efficacy. 8
- Clofazimine – conditional recommendation; useful when Group A agents are limited. 9
- Cycloserine or terizidone – conditional recommendation; added when other options are exhausted. 7
- Pyrazinamide – conditional, only if susceptibility confirmed. 5
- Ethambutol – conditional, reserved for cases where more potent drugs cannot be assembled. 5
5. Short‑Course BPaL/M Regimen for Selected MDR/RR‑TB
- Eligibility: patients without fluoroquinolone resistance, no prior >1 month exposure to second‑line drugs, and limited disease extent. 10
- Regimen: bedaquiline + pretomanid + linezolid (600 mg for 26 weeks) ± moxifloxacin.
- Outcomes: treatment success rates approach ~100 % in trial cohorts; linezolid 600 mg dosing yields significantly fewer adverse events than 1200 mg. 10
6. XDR‑TB Treatment Considerations
- Treatment duration: 15–24 months after culture conversion, modestly longer than MDR‑TB. 4
- Drug selection: when Group A agents are resistant, clinicians must incorporate Group B and Group C drugs. 12
- BPaL regimen (Nix‑TB study): bedaquiline + pretomanid + linezolid for 6 months achieved 90 % treatment success in XDR‑TB patients. 10
7. Global Outcomes
- Conventional MDR‑TB regimens achieve 50–88 % treatment success worldwide. 10
- BPaL‑based regimens raise success to ≈ 90 % even for XDR‑TB. 10
8. Diagnostic and Clinical Management Imperatives
- Rapid molecular testing for rifampin and isoniazid resistance is mandatory at diagnosis. 12
- Upon detection of rifampin resistance, immediate testing for fluoroquinolone and second‑line drug susceptibility is required. 8
- Exclude fluoroquinolone resistance whenever isoniazid resistance is identified, to avoid unnecessary regimen escalation. 8
- Empiric regimens without drug‑susceptibility testing are discouraged (except in culture‑negative TB) because they promote further resistance. 12
- Every drug in a regimen must have documented or presumed susceptibility based on testing or reliable epidemiologic data. 9
9. Expert Consultation
- Specialist consultation is essential for all drug‑resistant TB cases; in the United States, clinicians should access CDC‑supported TB Centers of Excellence or local health department expertise. 8