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

The newly developed E test was compared with an extended 1% proportion dilution method for determining the susceptibility of Mycobacterium avium complex (MAC) strains to amikacin, streptomycin, fusidic acid, rifampicin, clarithromycin, ciprofloxacin, ofloxacin and fleroxacin. For all antibiotics tested except clarithromycin and ciprofloxacin, no more than one strain gave a different susceptibility result with the two methods. The discrepant results occurred near the chosen breakpoint concentration of clarithromycin and outside the concentration range of the E test for ciprofloxacin. For the minimum inhibitory concentration (MIC) values obtained within the range of antibiotic concentrations tested, there was good correlation between the two methods; the MICs differed by more than one two-fold dilution in no more than two strains per antibiotic. It is concluded that the E test is suitable for susceptibility testing of MAC.