Comparison of the E test and a proportion dilution method for susceptibility testing of Mycobacterium tuberculosis

Minimal inhibitory concentrations (MICs) of amikacin, streptomycin, fusidic acid, rifampicin, clarithromycin, ciprofloxacin, ofloxacin, and fleroxacin were determined by the E test for 20 strains of Mycobacterium tuberculosis. The resulting discrimination in resistant or sensitive strains was compared with the results of an extended proportion dilution method. There were no more than three strains per antibiotic with different ratings with the exception of ciprofloxacin and ofloxacin. In these discrepant cases, the breakpoint concentrations had a position at the top of the test strip, which may be unfavourable for MIC reading. The MICs of streptomycin (1-2 mg/l) and rifampicin (2-4 mg/l) for the control strain M. tuberculosis H37Rv (ATCC 27,294) were close to the reference values according to the German standard DIN 58,943. It is concluded that the E test is suitable for susceptibility testing of slowly growing M. tuberculosis isolates.