Reassessment of the “Class” Concept of Disk Susceptibility Testing: Cephalothin Disks versus Minimal Inhibitory Concentrations with Eleven Cephalosporins

Reassessment of the "class" concept of disk susceptibility testing. Cephalothin disks versus minimal inhibitory concentrations with eleven cephalosporins. Am J Clin Pathol 70: 909--913, 1978. Studies were carried out to determine whether susceptibility or resistance to 11 cephalosporins could be predicted reliably from the results of tests with a single cephalothin disk. The cephalosporins were tested with a microdilution technic and with a standardized disk test. Strains susceptible to a cephalothin disk were predictably susceptible to all other cephalosporins. However, 2--12% of the strains were resistant to cephalothin disks but were susceptible to the more active parenteral drugs cefoxitin, cephamandole, cefuroxime, and BL-S786. Because of differences in antimicrobial activities, the cephalosporins could be divided into three subgroups for purposes of susceptibility testing: one subgroup includes the majority of cephalosporins and may be represented by tests with cephalothin, the second subgroup inclues three active parenteral drugs (cephamandole, cefuroxime, and BL-S786) and may be represented by tests with cefuroxime, and the third subgroup consists of cefoxitin, a cephamycin with a unique broad spectrum of activity. Until the drugs in the second and third subgroups are released for general therapeutic use, the practice of testing only one cephalosporin disk appears to be a reasonably reliable procedure.