Interpretive criteria for the agar diffusion susceptibility test with enoxacin

Abstract

Regression analyses to determine the correlation of MIC and inhibition zone produced by enoxacin discs were carried out using 300 freshly isolated cultures of infective organisms (20 strains each from 15 species). For this purpose, commercially available discs containing 10 micrograms enoxacin and locally made discs containing 5 and 10 micrograms enoxacin were used. Studies were performed simultaneously with I.C.S. and Kirby-Bauer methods on Iso-Sensitest and Mueller-Hinton agars. It was found that correlation becomes poorer with increasing disc content and when the Kirby-Bauer method was used. Based on preliminary MIC breakpoints of 1 and 4 mg/l and the zone cut-off points calculated from regression equations, no major errors were found in zone interpretations. Minor errors were observed in 3.7% with our own 5 micrograms discs used in the I.C.S. method, and in 11.0% with 10 micrograms commercial discs and the Kirby-Bauer method. For 5 micrograms enoxacin discs and the Kirby-Bauer method the following zone interpretations are recommended: resistant up to 13 mm, intermediate 14-21 mm, susceptible 22 mm or more. The respective values for the I.C.S. method are: resistant up to 14 mm, intermediate 15-22 mm, susceptible 23 mm or more.