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

Publisher Website
Google Scholar
Abstract
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 10μg norfloxacin disks were studied. Results were analyzed in terms of recently recommended breakpoints for clinical susceptibility (MIC ⩽ 16μg/ml, zone diameter ⩾ 17 mm) and resistance (MIC ⩾ 32μg/ml, zone diameter ⩽ 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 16μg/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.