Evaluation of alternative disk diffusion methods for detecting mecA-mediated oxacillin resistance in an international collection of staphylococci: Validation report from the SENTRY antimicrobial surveillance program

To validate the current National Committee for Clinical Laboratory Standards recommendations of the cefoxitin disk as a preferred surrogate marker to detect oxacillin resistance in staphylococcal isolates, 304 staphylococcal isolates originating from 49 sites in 16 countries in the SENTRY Antimicrobial Surveillance Program (2003) were tested. Two hundred three Staphylococcus aureus and 101 coagulase-negative staphylococci (CoNS), of which >95% were bloodstream isolates, were evaluated by comparing the results of the National Committee for Clinical Laboratory Standards broth microdilution method for oxacillin with those of the disk diffusion test using oxacillin, cefoxitin and ceftizoxime disks. Discrepancies were resolved using the PBP2a latex agglutination test. For S. aureus, the cefoxitin disk performed without interpretive error followed by the ceftizoxime disk (1% major and 0.5% minor errors; ≥20 mm = susceptible); use of the oxacillin disk test had the highest error rates with 4.4% major and 1.5% minor errors, whereas the oxacillin minimal inhibitory concentration (MIC) test was 99.0% accurate. For CoNS, the oxacillin disk test had the highest error rate with 4.0% major errors, followed by the cefoxitin (3.0% major error rate) and the ceftizoxime (1% very major and 1% minor error: ≥20 mm = susceptible) disk tests. The oxacillin MIC test was also 99.0% accurate for CoNS testing. Modification of the ceftizoxime disk diffusion breakpoints for CoNS resulted in complete intermethod categorical agreement. The overall accuracy of the four tests was as follows: modified ceftizoxime disk (99.3%) > oxacillin MIC = cefoxitin disk (99.0%) > current ceftizoxime disk (98.4%) > oxacillin disk (94.7%). In conclusion, these results confirm the superior performance characteristics of cefoxitin and ceftizoxime disk tests as surrogate markers to detect oxacillin resistance; by using an international collection of clinically significant staphylococcal isolates, we also demonstrate its wide global application.