[Criteria for the interpretation of sensitivity testing of ofloxacin with the agar diffusion test]

Regression analyses to determine the correlation of MIC and inhibition zone produced by ofloxacin disks were carried out using 300 freshly isolated cultures of infective organisms (20 strains each from 15 species). It was found in pilot studies that the correlation becomes poorer with increasing disk loads. Disks containing 5 micrograms of ofloxacin were chosen for comparative studies using Mueller-Hinton agar and Kirby-Bauer, as well as DIN-58940 methods. Based on preliminary MIC breakpoints of 1 and 4 mg/l and on calculations from regression equations, the following zone interpretations using the Kirby-Bauer method are recommended: resistant = up to 12 mm, intermediate = 13 to 19 mm, susceptible = 20 mm or more. The respective values for the DIN method are: resistant = up to 14 mm, intermediate = 15 to 21 mm, susceptible = 22 mm or more. The results are similar to those obtained by us in previous studies with norfloxacin, enoxacin and ciprofloxacin.