Minimal Inhibitory Concentrations of Rifabutin, Ciprofloxacin, and Ofloxacin against Mycobacterium tuberculosis Isolated before Treatment of Patients in Taiwan

Minimal inhibitory concentrations (MICs) of rifabutin, ciprofloxacin, and ofloxacin were determined for "wild" Mycobacterium tuberculosis strains, susceptible to all antituberculosis drugs in the conventional test, isolated from newly diagnosed Taiwanese patients who had never had prior treatment for tuberculosis. These MICs were within the limits previously reported for strains isolated in the United States. The range of MICs of rifabutin for Taiwanese strains was 0.015 to 0.125 micrograms/ml; ciprofloxacin, 0.25 to 2.0 micrograms/ml; ofloxacin, 0.5 to 2.0 micrograms/ml. On the basis of an evaluation of the highest broth-determined MICs found in this and in a previous study, we suggest that the following MICs, when determined in 7H12 broth radiometrically, should be used as breakpoints to classify the strain as "susceptible": for rifabutin, 0.125 micrograms/ml or less; for ofloxacin and ciprofloxacin, 2.0 micrograms/ml or less.