Disk diffusion susceptibility tests with norfloxacin: confirmation of proposed interpretive criteria

One hundred and eighty three clinical isolates of aerobic bacteria were tested against norfloxacin by both agar dilution (WHO-ICS) and disk diffusion test procedures (standardized FDA single disk test). Two experimental 10 microgram norfloxacin disks were studied. Results were analyzed in terms of recently recommended breakpoints for clinical susceptibility (MIC less than or equal to 16 micrograms/ml, zone diameter greater than or equal to 17 mm) and resistance (MIC greater than or equal to 32 micrograms/ml, zone diameter less than or equal to 12 mm). Excellent correlation was demonstrated by statistical analysis between paired MIC and zone size values (average value for each MIC; r = -0.9782). An MIC of 16 micrograms/ml was found to correlate with a zone of 10.4 mm. Application of the recommended zone size breakpoints resulted in prediction of 177 isolates as being susceptible while six (3.3%) were predicted to be either intermediate or resistant. The findings of this study validate the earlier recommendations stated above.