Evaluation of proposed criteria of disk susceptibility testing for tosufloxacin and lomefloxacin in NCCLS guidelines and WHO standards

Disk diffusion zone diameter breakpoint criteria for Tosufloxacin and Lomefloxacin were tentatively established by correlating MICs with 1-, 5- and 10 micrograms of Tosufloxacin disk inhibitory zone diameters and with 10 micrograms of Lomefloxacin disk of those to 418 clinical isolates representing 32 species. We recommend 5 micrograms disks for Tosufloxacin with the following breakpoints: Susceptible (MIC, less than or equal to 0.5 microgram/ml), greater than or equal to 22 mm; intermediate, 17 to 21 mm; and resistant (MIC, greater than or equal to 2.0 micrograms/ml), less than or equal to 16 mm. We recommend 10 micrograms disks for Lomefloxacin with the following breakpoints: Susceptible (MIC, less than or equal to 2.0 micrograms/ml), greater than or equal to 21 mm; intermediate, 16 to 20 mm; and resistant (MIC, greater than or equal to 8.0 micrograms/ml), less than or equal to 15 mm. Using these criteria for Tosufloxacin and Lomefloxacin, the occurrence rate of major errors in judging susceptibility and resistance was 0.48%.