Reliability of cefaclor, cefazolin, cefamandole, and cephalothin disks to predict susceptibility of Enterobacteriaceae, Staphylococcus species, and Haemophilus influenzae.

Interpretive zone-size standards currently used for cephalothin and cefamandole disk tests also may be applied to tests with disks containing 30 micrograms of cefaclor or cefazolin. Against 627 representative isolates, susceptibility to cefaclor and cefazolin could be predicted by testing cephalothin. However, cefazolin is more active than cephalothin against isolates of Escherichia coli with a TEM beta-lactamase plasmid. The expanded spectrum of cefamandole continues to necessitate separate testing. Against methicillin-resistant staphylococci, cefaclor disks were more reliable than cephalothin or cefamandole, but false-susceptible results were seen with all four disks. For testing Haemophilus influenzae, the cefazolin disks were not reliable; cephalothin or cefaclor disks could predict susceptibility to either drug.