[A study of the disc sensitivity test for cephradine].

Susceptibilities of 189 strains of 28 bacterial species to cephradine were determined by the 2-fold agar dilution method in parallel with the diameter of inhibition zone by the single-disc method, under the experimental condition established by Kanazawa. The experiments demonstrated significant correlation between MIC by the dilution method and diameter of inhibition zone in each of conventional assay of the over-night (about 16 hours) incubation, delayed assay (about 24 hours incubation), and rapid assay (after 3-4 or 5-6 hours incubation), thus confirming applicability of the single-disc assay for cephradine. Analysis of the data obtained by using cephradine disc containing 30 micrograms revealed the primary regression equation to be: D (diameter, mm) = 28.5-11.2 log MIC (microgram/ml) in conventional assay, D = 35.1-14.6 log MIC (microgram/ml) in delayed assay, D = 18.0-5.41 log MIC (microgram/ml) in 3-4 hours rapid assay and D = 23.2-8.33 log MIC (microgram/ml) in 5-6 6 hours rapid assay, respectively. The range of variations in MICs estimated from the diameter of of inhibition zone by the disc test was then calculated in comparison with that in MIC determined by the 2-fold agar dilution assays, as reference for the experimental errors which may be involved in the estimation of MIC of cephradine by the single-disc assay.