Pseudo-outbreak of methicillin-resistant Staphylococcus aureus

Objective: To determine whether a high rate of methicillin-resistant Staphy lococcus aureus at our institution was due to laboratory misclassification and to evaluate the effect of this misclassification. Material and Methods: We evaluated all S aureus isolates identified at our institution during a 60-day period in 1997, Automated susceptibility test r esults (using the Vitek system) from our clinical microbiology laboratory a nd an independent laboratory were compared with oxacillin agar screen plate results at both laboratories. Isolates with discordant results for suscept ibility to oxacillin were tested by broth microdilution minimal inhibitory concentrations and for the presence of the mecA gene. Results: Eighteen (72%) of the 25 organisms (obtained from 17 patients) fou nd to be resistant to oxacillin by the Vitek system at our institution were susceptible by the oxacillin agar screen. Discordant isolates tested by br oth microdilution minimal inhibitory concentrations and for the mecA gene w ere found to be oxacillin susceptible and mecA gene negative. Thus, at our hospital, almost three fourths of the organisms initially identified as met hicillin-resistant S aureus by the Vitek system were actually susceptible t o oxacillin, This misclassification resulted in needless infection control measures and unnecessary vancomycin use. Conclusion: Hospitals that use only automated susceptibility testing for S aureus should periodically validate their results with additional testing.