Affiliation:
1. Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia 30333
Abstract
ABSTRACT
The cefoxitin disk diffusion (DD) test for predicting
mecA-
mediated oxacillin resistance in staphylococci was assessed during a three-phase study. In phase 1, one laboratory tested 62 and 53 strains of
Staphylococcus aureus
and coagulase-negative staphylococci (CoNS), respectively. These data were used to choose the provisional cefoxitin DD breakpoints (resistant/susceptible) of ≤19 mm/≥20 mm for
S. aureus
and ≤24 mm/≥25 mm for CoNS for the next phase of testing. In phase 2, 10 laboratories each tested approximately 40 in-house strains of staphylococci (half of which were
S. aureus
) using Mueller-Hinton agar from different manufacturers. In this phase, the sensitivity and specificity, respectively, of the cefoxitin disk test were 98 and 100% for
S. aureus
and 99 and 96% for CoNS. The cefoxitin DD test performed equivalently to oxacillin broth microdilution (BMD) and to oxacillin DD tests among
S. aureus
and
mecA
-positive CoNS strains but gave better results than oxacillin BMD or oxacillin DD for
mecA
-negative strains of CoNS. The cefoxitin DD test also was much easier to read and did not require the use of transmitted light for detection of resistance. Based on data from the first two phases, the Clinical and Laboratory Standards Institute (CLSI; formerly NCCLS) adopted the use of the cefoxitin DD test for predicting
mecA-
mediated oxacillin resistance in staphylococci and revised Table 2C in CLSI document M100-S14 to reflect the change. In the third phase, an additional 61 challenge strains of CoNS for which the oxacillin MICs were 0.5 to 2 μg/ml were tested in a single laboratory to determine the effectiveness of the cefoxitin DD test for this group of borderline-resistant isolates. These data were used to refine the description of the test in CLSI document M100-S15. The cefoxitin DD test is preferred over the oxacillin DD test for predicting
mecA
-mediated oxacillin resistance in
S. aureus
and CoNS.
Publisher
American Society for Microbiology
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