Comparison of cefoxitin and moxalactam 30 µg disc diffusion methods for detection of methicillin resistance in coagulase-negative staphylococci

OBJECTIVES: To compare cefoxitin and/or moxalactam 30 microg disc diffusion (DD) methods to detect methicillin resistance in coagulase-negative staphylococci (CoNS) using both high- and low-density (HD/LD) inoculum techniques. METHODS: A challenge set of 192 CoNS was tested. DD test results were compared with PBP2a detection. RESULTS: With the LD inoculum, the sensitivity/specificity of cefoxitin and moxalactam were 94.4%/100% and 100%/92.4%, respectively, using the DD breakpoints of the Comité de l'Antibiogramme de la Société Française de Microbiologie. With the HD inoculum, the sensitivity/specificity of cefoxitin and moxalactam were 93.7%/100% and 100%/96.9%, using the cefoxitin DD breakpoints of the CLSI and a resistant/susceptible breakpoint of < 20 mm/>or=20 mm for moxalactam. Comparison of receiver operating characteristic AUCs did not show significant difference between studied assays, but the overlapping zone where both PBP2a-positive and PBP2a-negative isolates were observed concerned a lower number of strains with moxalactam than with cefoxitin (P < 0.001). Combination of cefoxitin and moxalactam DD methods demonstrated that all isolates with a concordant cefoxitin/moxalactam phenotype were correctly classified. Interestingly, all isolates misclassified by each DD method used alone were cefoxitin-susceptible and moxalactam-resistant. CONCLUSIONS: Although all DD methods studied here performed well for detecting methicillin resistance in CoNS, moxalactam had a higher accuracy than cefoxitin to differentiate heteroresistant isolates from PBP2a-negative strains. Identification of isolates that should be submitted to a confirmatory test to conclude on methicillin resistance can be easily obtained by combining cefoxitin and moxalactam DD methods.