Reevaluations of disk diffusion susceptibility testing interpretive criteria for lomefloxacin and norfloxacin using fluoroquinolone-resistant isolates

Abstract

The current National Committee for Clinical Laboratory Standards interpretive criteria for disk diffusion susceptibility testing of lomefloxacin and norfloxacin were reevaluated using a test panel of 298 bacteria (200 with ciprofloxacin minimum inhibitory concentrations (MICs) at ⩾4 μg/ml). MICs were correlated with the diameter of zones of inhibition by regression statistics and error-rate bounding methods. Modifications of the interpretive criteria for lomefloxacin disk susceptibility testing are proposed as follows: susceptible at ⩾20 mm and resistant at ⩽16 mm, a 2-mm decrease of existing break-point zones. These criteria 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 ⩾16 mm and resistant at ⩽13 mm) were validated, and these break-points had an absolute interpretive correlation between methods of 91.9%. The change proposed for lomefloxacin disk interpretations would minimize minor and major errors most often reported for Staphylococcus saprophyticus isolates.