Interpretive criteria for the agar diffusion susceptibility test with ofloxacin

Regression analyses to determine the correlation of minimal inhibitory concentrations (MICs) and inhibition zones produced by ofloxacin discs were carried out with 300 freshly isolated cultures of infective organisms (20 strains from each of 15 species). After pilot studies, 5 micrograms loaded discs were chosen. Studies were performed simultaneously by using ICS/DIN and Kirby-Bauer/NCCLS methods on Mueller-Hinton agar. It was found that the correlation becomes poorer with increasing disc loads and when the Kirby-Bauer method is used. Based on preliminary MIC breakpoints of greater than or equal to 2 mg/L and greater than or equal to 8 mg/L and by calculations from regression equations, the following zone interpretations using the NCCLS method are recommended: resistant up to 12mm, intermediate 13 to 15mm, susceptible 16mm or more. The values for the DIN method are: resistant up to 13mm, intermediate 14 to 17mm, susceptible 18mm or more. No major errors were found in zone interpretations. Minor errors were observed in 0.7% when the NCCLS method was used, and in 1.3% with the DIN method.