The World Health Organization (WHO) has updated the critical concentrations for culture-based phenotypic drug susceptibility testing (DST) to anti-tuberculosis drugs.

The agency has reduced the critical concentrations for rifampicin; however, that for isoniazid continues to be at the current level. The revision in the critical concentrations will address the difference between the phenotypic and molecular methods for detection of resistance to rifampicin and will help enhance the accuracy of DST.

The updates have been proposed following the meeting of a Technical Expert Group in 2020 to evaluate the findings of a systematic review of published literature concerning the critical concentrations for DST of the key first-line anti-TB drugs, including isoniazid and the rifamycins. It has been observed that critical concentrations used for phenotypic methods for identifying rifampicin resistance may inappropriately characterize the strains with certain mutations… (WHO, February 5, 2021)