What is Extensively Drug-Resistant (XDR) Tuberculosis?
XDR-TB is defined as multidrug-resistant tuberculosis (MDR-TB) with additional resistance to any fluoroquinolone AND either bedaquiline or linezolid (or both). This updated definition, established in 2021, replaced the older definition that included resistance to second-line injectable drugs 1.
Evolution of the XDR-TB Definition
The definition of XDR-TB underwent a critical revision in 2020-2021 based on evidence showing that second-line injectable drugs (SLIDs) like amikacin, kanamycin, and capreomycin were no longer central to effective MDR-TB treatment 1. The previous definition (pre-2020) classified XDR-TB as MDR-TB plus resistance to any fluoroquinolone and any SLID 2, 3, 4.
The new evidence-based definition focuses on resistance to the most critical drugs in modern MDR-TB regimens:
- Baseline: Resistance to rifampicin and isoniazid (MDR-TB)
- Plus: Resistance to any fluoroquinolone (levofloxacin or moxifloxacin)
- Plus: Resistance to bedaquiline and/or linezolid 1
Why the Definition Changed
The WHO substantially limited the role of SLIDs in MDR-TB treatment starting in 2019, instead prioritizing Group A drugs (fluoroquinolones, bedaquiline, and linezolid) for longer regimens 5. Evidence from 11,666 patients demonstrated that:
- Fluoroquinolone resistance nearly doubled the odds of unfavorable treatment outcomes (adjusted OR 1.91) 1
- Administration of bedaquiline and/or linezolid dramatically improved outcomes even in patients with XDR-TB 1
- Among XDR-TB patients receiving no Group A drugs, those who received linezolid only had 63% lower odds of unfavorable outcomes (aOR 0.37), bedaquiline only had 60% lower odds (aOR 0.40), and both together had 79% lower odds (aOR 0.21) 1
Clinical Implications
This definition matters because it identifies patients who have resistance to the most effective drugs currently available for MDR-TB treatment. The WHO now strongly recommends that fluoroquinolones, bedaquiline, and linezolid should all be included in longer MDR-TB regimens when susceptibility is confirmed 5.
Treatment Challenges
XDR-TB remains extremely difficult to treat with success rates historically around 30-50%, and even poorer outcomes in HIV-infected patients 2. The disease requires 18-24 months of treatment with four to six second-line drugs 2, though newer shorter regimens (6-month BPaL/BPaLM) are now recommended for eligible patients 6.
Key Caveat
The updated definition specifically supports surveillance and treatment choice in the era of bedaquiline- and linezolid-containing regimens 1. It reflects the reality that resistance to these newer agents represents the most serious threat to successful MDR-TB treatment outcomes.