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

Abstract

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 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.

How a PCR Sequencing Strategy Can Bring New Data to Improve the Diagnosis of Ethionamide Resistance

“…In light of these results, carefully analyzing the strains without genotypic/phenotypic correlation is of great interest. The 26 ETH-S isolates with at least one alteration, phylogenetic SNPs excluded ( Table 2 ), may be explained by the fact that (i) the genetic alteration observed is not implicated in ETH-R, (ii) the strains have a low level of ETH-R which is not detected by the phenotypic DST [ 33 , 34 , 35 , 36 ], or (iii) compensatory mutations restore ETH susceptibility. Other flavin monooxygenases such as EthA2 or MymA are involved in ETH activation [ 24 , 37 ].…”

“…This is similar to what has been described for ethambutol, supporting the idea that the reporting of strains with MICs close to the ECOFF could be affected by reproducibility issues, as the classification into susceptible or resistant highly depends on methodological variation [ 34 ]. Third, ETH DST has been shown to have poor concordance and reproducibility compared to other drugs [ 35 , 36 ]. As previously suggested, creating an intermediate susceptibility classification for ETH-S strains with gene alteration, supported by the unfavorable pharmacodynamic indices, could be warranted to increase reproducibility and to account for methodological variation [ 34 ].…”