Cephalexin susceptibility breakpoint for veterinary isolates: Clinical Laboratory Standards Institute revision

The Clinical and Laboratory Standards Institute (CLSI) uses cephalothin as the class representative for testing veterinary isolates for susceptibility to other first-generation cephalosporins, including cephalexin. We examined replacing cephalothin with cephalexin because cephalexin is used more often clinically. Bacterial isolates were obtained from dogs and cats from a national surveillance program. CLSI testing methods were used to determine the MIC for 4 cephalosporins used in veterinary medicine. Cephalexin clinical breakpoints for canine isolates were established by using published pharmacokinetic data and Monte Carlo simulations to calculate the probability of target attainment (PTA). For 1,112 Staphylococcus pseudintermedius isolates, the mode, MIC₅₀, and MIC₉₀ were 1, 2, and 64 µg/mL, respectively, for cephalexin, and ≤0.06, 0.12, and 2 µg/mL for cephalothin. Susceptibility of S. pseudintermedius from 2011 to 2014 did not change for the 4 cephalosporins tested. Only 4.3% of the penicillin-binding protein 2a-positive S. pseudintermedius isolates had MIC values ≤2 µg/mL for cephalexin, but 66.3% of these isolates had MIC values ≤2 µg/mL for cephalothin. There were also discrepancies between cephalexin and cephalothin for other bacteria tested, but the largest difference was for S. pseudintermedius, with a MIC difference of 4 doubling dilutions. Cephalexin interpretive categories (breakpoints) of ≤2 μg/mL (susceptible), 4 μg/mL (intermediate), and ≥8 μg/mL (resistant) were established for isolates obtained from dogs. Cephalothin should not be used for susceptibility testing of cephalexin for veterinary bacterial pathogens, and canine-specific breakpoints should be used for testing susceptibility. Breakpoints determined using the methods described herein for the interpretive categories will be added to future CLSI tables to reflect this recommendation.