Multicentre study to establish interpretive criteria for clofazimine drug susceptibility testing

OBJECTIVE: To conduct a multicentre study to establish the critical concentration (CC) for clofazimine (CFZ) for drug susceptibility testing (DST) of Mycobacterium tuberculosis on the MGIT (TM) 960 (TM) system using the distribution of minimum inhibitory concentrations (MIC) and genotypic analyses of Rv0678 mutations.

DESIGN: In phase I of the study, the MIC distribution of laboratory strains (H37Rv and in vitro-selected Rv0678 mutants) and clinical pan-susceptible isolates were determined (n = 70). In phase II, a tentative CC for CFZ (n = 55) was proposed. In phase III, the proposed CC was validated using clinical drug-resistant tuberculosis (DR-TB) isolates stratified by Rv0678 mutation (n = 85).

RESULTS AND CONCLUSION: The MIC distribution of CFZ for laboratory and clinical pan-susceptible strains ranged between 0.125 mu g/ml and 0.5 mu g/ml. As the MIC values of DR-TB isolates used for phase II ranged between 0.25 mu g/ml and 1 mu g/ml, a CC of 1 mu g/ml was proposed. Validation of the CC in phase III showed that probably susceptible and probably resistant Rv0678 mutants overlapped at 1 mu g/ml. We therefore recommend a CC of 1 mu g/ml, with additional testing at 0.5 mu g/ml to define an intermediate category. This was the first comprehensive study to establish a CC for routine phenotypic DST of CFZ using the MGIT960 system to guide therapeutic decisions.