Revisiting the susceptibility testing of Mycobacterium tuberculosis to ethionamide in solid culture medium.

Background & objectives: Increase in the isolation of drug resistant phenotypes of Mycobacterium tuberculosis necessitates accuracy in the testing methodology. Critical concentration defining resistance for ethionamide (ETO), needs re-evaluation in accordance with the current scenario. Thus, re-evaluation of conventional minimum inhibitory concentration (MIC) and proportion sensitivity testing (PST) methods for ETO was done to identify the ideal breakpoint concentration defining resistance. Methods: Isolates of M. tuberculosis (n=235) from new and treated patients were subjected to conventional MIC and PST methods for ETO following standard operating procedures. Results: With breakpoint concentration set at 114 and 156 μg/ml, an increase in specificity was observed whereas sensitivity was high with 80 μg/ml as breakpoint concentration. Errors due to false resistant and susceptible isolates were least at 80 μg/ml concentration. Interpretation & conclusions: Performance parameters at 80 μg/ml breakpoint concentration indicated significant association between PST and MIC methods.

Optimization of the conventional minimum inhibitory concentration method for drug susceptibility testing of ethionamide

Evaluation of newer methods and optimization of existing methods for the susceptibility testing of second-line drugs, especially ethionamide, are essential when treatment of multidrug-resistant tuberculosis [MDR-TB] is warranted. The ideal method must clearly demarcate sensitive from resistant strains. Hence, optimization of the conventional minimum inhibitory concentration [MIC] method was attempted using diluted inoculum. The optimized MIC method was evaluated using 206 Mycobacterium tuberculosis strains isolated from new and previously treated tuberculosis patients and were compared with the conventional MIC method and proportion sensitivity [PST] method. The sensitivity and specificity of the optimized MIC method in comparison with the PST method was 74% and 90%. Assessment of the optimized MIC method with the conventional MIC method gave a sensitivity of and specificity of 73% and 98%. Overall agreement between the 2 methods was found to be >/= 80%. Endowed with the ability to identify the resistant strains precisely, the optimized MIC method can be used for screening resistance to ethionamide