Determination of minimal inhibitory concentrations of antituberculosis drugs by radiometric and conventional methods

Minimal inhibitory Concentrations (MIC) of 5 antituberculosis drugs (isoniazid, rifampin, streptomycin, ethionamide, and ethambutol) were determined by the radiometric (BACTEC) broth method and by the agar plate proportion method. Seventeen M. tuberculosis strains, isolated from patients before treatment, were tested. The MIC values of 4 of the 5 drugs (the exception was streptomycin) were 2 to 4 times lower in 7H12 broth than in 7H11 agar. The broth-determined MIC were also at least 2 to 4 times lower than the achievable serum concentrations. The broth-determined MIC are probably much closer to the true MIC values than those determined in agar plates because of the lower degree of absorption and degradation in the liquid medium. The radiometric broth method is a simple and rapid quantitative method for accurate determination of the MIC values of the antituberculosis drugs. The data obtained in this study will be used for further evaluation of the MIC values as complementary or alternative to the conventional qualitative testing against critical concentrations.