Evaluation of the broth microdilution methodology for susceptibility testing of Mycobacterium tuberculosis in Peru

Peru is amongst the 30 countries with the highest burden of multidrug-resistant tuberculosis worldwide. In the fight against drug-resistant tuberculosis, the UKMYC6 microtiter plate was developed and validated by the CRyPTIC project. Our objective was to evaluate the use of the broth microdilution UKMYC6 plate for susceptibility testing of drug-resistant Mycobacterium tuberculosis (MTB) strains in Peru. 496 nationally-representative MTB strains determined drug-resistant by the routine agar proportion method (APM) were selected. MICs of 13 anti-tuberculosis drugs were determined for each strain using the microdilution UKMYC6 plates and compared with the APM result. MIC distributions for APM-susceptible and APM-resistant strains were demonstrated for rifampicin, isoniazid, kanamycin, and levofloxacin, with reasonable agreement (0.64[≤]k[≤]0.79) for rifampicin, ethambutol, ethionamide and kanamycin and the best agreement for isoniazid and levofloxacin (k>0.8). No strain presented MICs higher than the CRyPTIC Epidemiological cut-off values for the new (bedaquiline, delamanid) or repurposed (clofazimine, linezolid) drugs. The microbroth dilution method using the UKMYC6 microtiter plate allowed the complete susceptibility characterization, through the determination of MICs, of drug-resistant Mycobacterium tuberculosis strains in Peru. This methodology showed a good diagnostic performance for the drugs rifampicin, isoniazid, kanamycin and levofloxacin drugs.