Tentative interpretive standards for agar disk diffusion antimicrobial susceptibility testing of cefoperazone.

Cefoperazone is a new cephalosporin with a very wide spectrum of activity, including activity against Pseudomonas aeruginosa. It has less activity on enterococci and Acinetobacter. Of the 459 selected bacterial strains tested in this study, only 1.5% (7 strains and 6 genera) had minimum inhibitory concentrations of greater than or equal to 128 micrograms/ml. For a minimum inhibitory concentration breakpoint of less than or equal to 32 micrograms/ml (susceptible), we recommend that the disk diffusion test be done with a 75-micrograms disk and breakpoints of greater than or equal to 18 mm for susceptible, 15 to 17 mm for intermediate, and less than or equal to 14 mm for resistant. Diffusion tests using these criteria yielded only 1.1% very major or major errors.