Last week, the World Health Organization (WHO) held a consultation to discuss the definition of extensively drug resistant (XDR) tuberculosis. The meeting was held to discuss novel treatment regimens and diagnostics for drug-resistant TB and determine its impact on the definition of XDR-TB. The meeting was held virtually with over 70 participants with country representatives, international organizations, agencies, NGOs, civil society, and academia. The meeting was held with the intention of updating the definition to provide guidance that would be simple, measurable, and relevant to national TB programs.
The consultation participants determined that the definition of MDR-TB would remain the same. A new definition for "pre-XDR-TB" was created separate of XDR-TB. The new WHO definitions are as follows:
Pre-XDR-TB: TB caused by Mycobacterium tuberculosis (M.tuberculosis) strains that fulfill the definition of MDR/RR-TB and which are also resistant to any fluoroquinolone*
XDR-TB: TB caused by Mycobacterium tuberculosis (M. tuberculosis) strains that fulfill the definition of MDR/RR-TB and which are also resistant to any fluoroquinolone and at least one additional Group A drug*
*The fluoroquinolones include levofloxacin and moxifloxacin as they are the fluoroquinolones currently recommended by WHO for inclusion in longer regimens. The Group A drugs are currently levofloxacin or moxifloxacin, bedaquiline and linezolid, therefore XDR-TB is MDR/RR-TB that is resistant to a fluoroquinolone and at least one of bedaquiline or linezolid (or both). The Group A drugs may change in the future; therefore the terminology Group A is appropriate here and it will apply to any Group A drugs in the future.
These definitions should be applied from January 2021.
For more information on the WHO's newest definitions of pre-XDR-TB and XDR-TB, visit their online press release here: https://www.who.int/publications/i/item/meeting-report-of-the-who-expert-consultation-on-the-definition-of-extensively-drug-resistant-tuberculosis