[Interpretive compatibility of antimycobacterial susceptibility for Mycobacterium tuberculosis determined by...

The antimycobacterial susceptibility test method newly proposed by the Japanese Society for Tuberculosis, a proportion method on egg-based Ogawa media, was evaluated in comparison with microdilution test for Mycobacterium tuberculosis complex, BrothMIC MTB-1 (Kyokuto Pharmaceutical Inc., Tokyo). In the evaluation, five antimicrobial agents, streptomycin, ethambutol, kanamycin, isoniazid and rifampicin were included. Through repeated testings of the three reference strains against five antimicrobial agents, both test methods were found to be highly precise. All the minimum inhibitory concentrations (MICs) determined by BrothMIC MTB fell within 3 log2 dilutions, however a total of 11 MICs resulted in indeterminate(I) interpretations. Whereas, all the test results by a proportion method on Ogawa media were comparable to the expected interpretations. However, three of 48 testings resulted in undeterminable interpretations due to insufficient growth on the growth control media. A total of 127 clinical isolates of M. tuberculosis complex were tested by both methods, and 89 to 90% of the test results were comparable with each other in category interpretations. However, 7.1 to 9.4% of MICs determined by BrothMIC MTB resulted in indeterminate(I), and 0.8 to 3.1% of discrepant interpretations were observed. In conclusion, both test methods were highly precise and comparable in determining antimycobacterial susceptibility for M. tuberculosis complex. Several advantages and disadvantages in each test method were discussed.