[A study on the disc sensitivity test for cefpirome]

Susceptibilities of 232 strains of 40 bacterial groups to cefpirome (CPR) were determined by the 2-fold agar dilution method in parallel with the diameter of inhibition zones by the single-disc method under the experimental conditions established by Kanazawa. The experiments demonstrated a significant correlation between MIC by the dilution method and diameter of inhibition zone in each of the conventional assay of over-night (about 16 hours) incubation, the delayed assay (about 24 hours incubation), and the rapid assay (about 3-4 or 5-6 hours incubation), thus confirming the applicability of the single-disc assay for CPR. Analysis of the data obtained by using CPR disc containing 30 micrograms revealed the primary regression equation to be: D (diameter, mm) = 26.7-9.2 log MIC (micrograms/ml) in the conventional assay, D = 33.8-12.7 log MIC (micrograms/ml) in the delayed assay, D = 21.2-6.7 log MIC (micrograms/ml) in 5-6 hours rapid assay, and D = 14.8-4.1 log MIC (micrograms/ml) in 3-4 hours rapid assay. The range of variations in MICs estimated from the diameter of inhibition zone by the disc test was then calculated in comparison with that in MIC determined by the 2-fold agar dilution test, as a reference for the experimental errors which may be involved in the estimation of MIC of CPR by the single-disc assay.