[A study of INH 0.1 microgram/ml resistant M. tuberculosis strains assessed by BrothMIC MTB-1 method].

In the antimycobacterial susceptibility test for INH using the egg-based Ogawa media, 3 concentrations (0.1, 1, or 5 micrograms/ml) of INH were used, and 1 microgram/ml was used as a critical concentration for INH resistance. However, it was controversial whether INH 0.1 microgram/ml resistant M. tuberculosis was clinically significant or not. We investigated the MIC values of INH 0.1 microgram/ml resistant strains by using BrothMIC MTB-1 method, and 115 strains of M. tuberculosis confirmed by DNA-prove test were used. The distribution of MIC values of 115 strains determined by Ogawa INH susceptibility test was shown in figure. By BrothMIC MTB-1 method, they were classified into 3 groups; susceptible, low resistant and high resistant groups. The mean MIC value of INH 0.1 microgram/ml resistant M. tuberculosis was estimated to be 4.53 micrograms/ml with its 95% confidence interval 3.21-5.85 micrograms/ml, and they were determined as "resistant" in BrothMIC MTB-1 method. These results supported the idea that patients with INH 0.1 microgram/ml resistant M. tuberculosis strains should be regarded as clinically "resistant".