REEVALUATIONS OF DISK DIFFUSION SUSCEPTIBILITY TESTING INTERPRETIVE CRITERIA FOR LOMEFLOXACIN AND NORFLOXACIN USING FLUOROQUINOLONE-RESISTANT ISOLATES

The current National Committee for Clinical Laboratory Standards inter pretive criteria for disk diffusion susceptibility testing of lomeflox acin and norfloxacin were reevaluated using a test panel of 298 bacter ia (200 with ciprofloxacin minimum inhibitory concentrations (MICs) at greater than or equal to 4 mu g/ml). MICs were correlated with the di ameter of zones of inhibition by regression statistics and error-rate bounding methods. Modifications of the interpretive criteria for lomef loxacin disk susceptibility testing are proposed as follows: susceptib le at greater than or equal to 20 mm and resistant at less than or equ al to 16 mm, a 2-mm decrease of existing break-point zones. These crit eria result in an absolute interpretive agreement of 95.3% with a very major (false susceptible) error rate of only 0.7%. The currently used criteria for norfloxacin (susceptible at greater than or equal to 16 mm and resistant at less than or equal to 13 mm) were validated, and t hese break-points had an absolute interpretive correlation between met hods of 91.9%. The change proposed for lomefloxacin disk interpretatio ns would minimize minor and major errors most often reported for Staph ylococcus saprophyticus isolates.