Cefuroxime, cefamandole, cefoxitin, and cephalothin in vitro susceptibility tests: reassessment of the "class representative" concept, confirmation of disk interpretive criteria, and proposed quality control guidelines.

The relationship of cefuroxime in vitro susceptibility tests to similar cephalosporins (cefamandole, cefoxitin, and cephalothin) was evaluated using 396 recent clinical isolates. The previously published interpretive criteria of greater than or equal to 18 mm (less than or equal to 8.0 micrograms/mL) = susceptible and less than or equal to 14 mm (greater than or equal to 32 micrograms/mL) = resistant for each drug were considered appropriate. The results of all study methods demonstrated cefuroxime to be slightly less active than cefamandole against most species, yet both drugs possessed nearly identical antimicrobial spectra. Cephalothin and cefoxitin were confirmed to have spectra significantly different from cefamandole and from each other, thus requiring separate testing. The application of the "class representative" concept to cefuroxime and cefamandole seems justified. Use of a 30-micrograms cefuroxime disk yielded the best predictive results and minimized the number of false-susceptible (very major) interpretive errors. Quality control guidelines are presented in a tentative form for cefuroxime, and modifications in the cephalothin and cefamandole zone limits are suggested.