In vitro studies with cefotaxime: disk diffusion susceptibility tests.

Abstract

Until recently two sets of conflicting interpretive criteria existed for use with the standardized 30-mug cefotaxime diffusion disk [National Committee for Clinical Laboratory Standards (NCCLS) M2-A2S, supplement 1; product information package insert for Claforan (cefotaxime sodium), edition 10/81.] The latter criteria recently were superseded by a third set which differs radically from the first two in both minimal inhibitory concentration (MIC) and zone size breakpoints. The first two sets of criteria differed mainly in the zone of inhibition diameters used to predict resistant and intermediate organisms. The accuracies of these two sets of criteria were evaluated by paired agar dilution (World Health Organization-International Collaborative Study) and disk diffusion tests (Food and Drug Administration) with 347 clinical isolates of aerobic bacteria. A total of 274 isolates (79%) were clinically susceptible by agar dilution as defined by NCCLS (MIC </=8 mug/ml) and 61, including 48 Pseudomonas spp., were of intermediate susceptibility (MIC of 16 or 32 mug/ml). Twelve were resistant (MIC >/=64 mug/ml). The original product information package insert criteria proved to be most unreliable in identification of the intermediate organisms (zone of inhibition diameters of 18 to 22 mm); only 5 were correctly predicted as being intermediate, whereas 54 were predicted as being resistant, and 2 were predicted as being susceptible. In contrast, the NCCLS criteria (zone of inhibition diameter of 15 to 22 mm) predicted 41 as being intermediate and 18 as being resistant; again, 2 were susceptible. The 12 isolates resistant to cefotaxime by agar dilution were correctly predicted to be resistant by either set of breakpoints (zone of inhibition diameter of </=14 or </=17 mm). These results suggest that the package insert criteria previously recommended for use with the 30-mug cefotaxime disk were overly conservative and required revision to bring them into agreement with NCCLS performance standards. Unfortunately however, the recent revision of the package insert criteria has further confused this issue [product information package insert for Claforan (cefotaxime sodium), edition 2/82].