WHO announces updated critical concentrations for susceptibility testing for rifampicin

5 February 2021 | GENEVA – The critical concentrations for culture-based phenotypic drug susceptibility testing (DST) to first-line anti-TB drugs have been revised by the the World Health Organization (WHO). Critical concentrations for rifampicin have been lowered while those for isoniazid have been maintained at the present level. This update helps address the discordance observed between phenotypic and molecular methods to detect rifampicin resistance and improves the accuracy of DST. As a result patients with TB will have a more accurate  diagnosis.

The revision was the outcome of a Technical Expert Group meeting convened by WHO in 2020 to assess the results of a systematic review of published literature on critical concentrations for DST of the most important first-line anti-TB drugs, isoniazid and the rifamycins (rifampicin, rifabutin and rifapentine). New evidence showed that critical concentrations used for phenotypic methods to detect rifampicin resistance may incorrectly classify strains with certain mutations. The following media were considered: Löwenstein-Jensen (LJ), Middlebrook 7H10 (7H10), Middlebrook 7H11 (7H11) and BACTEC™ Mycobacterial Growth Indicator Tube™ 960 (MGIT).Technical Report on critical concentrations for drug susceptibility testing of isoniazid and the rifamycins (rifampicin, rifabutin and rifapentine)

Guidance has been provided to resolve discordance between genotypic and phenotypic results for these drugs and areas for further research have been highlighted. The full report is available here.

DST methods continue to have a very important role to identify resistance not detected by molecular assays and to support the interpretation of molecular assays results. However, they require sophisticated laboratory infrastructure, qualified staff and strict quality assurance procedures.

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