JAC Advance Access published online on March 5, 2007
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkl548
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Comparison of cefoxitin and moxalactam 30 µg disc diffusion methods for detection of methicillin resistance in coagulase-negative staphylococci
1 Microbiology Laboratory 2 Biostatistics Department, Necker-Enfants Malades Hospital, Faculté de Médecine Paris—Descartes, 149 rue de Sèvres, 75015, Paris, France
Received 5 October 2006; returned 2 November 2006; revised 1 December 2006; accepted 17 December 2006
* Corresponding author. Tel: +33-1-44-49-49-61; Fax: +33-1-44-49-49-60; E-mail: join{at}necker.fr
Objectives: To compare cefoxitin and/or moxalactam 30 µg 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/
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.
Key Words: heteroresistance , PBP2a , conditional logistic regression
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