DETECTION OF OXACILLIN-RESISTANCE IN STAPHYLOCOCCUS-AUREUS BY MICROSCAN MIC PANELS IN COMPARISON TO 4 OTHER METHODS

Two hundred fifty-two isolates of Staphylococcus aureus were tested fo r oxacillin susceptibility by MicroScan Cram positive overnight and ra pid MIC panels. Results were compared with nonautomated methods includ ing disk diffusion, MRSA Crystal ID, and Etests using MRSA Screen Agar as reference. One hundred sixty-nine isolates (67.1%) were oxacillin- susceptible and 83 (32.9%) were resistant. All methods agreed for 234 (92.9%) isolates. Very major error rates were 1.2% for disk diffusion, 3.6% for Etest, and 0 for all other methods. Major error rates were 5 .3% for MicroScan overnight panels, 3% for rapid panels, 2.4% for disk diffusion, 1.2% for Etest, and 0.6% for MRSA Crystal ID. Nine oxacill in-susceptible isolates with borderline MICs and discrepant results fo r 1 or more methods were tested for the mec A gene and all were negati ve. Each was susceptible to beta lactam/beta lactamase inhibitor combi nations, suggesting that false resistance may have been due to excessi ve beta lactamase production. Oxacillin-resistant S. aureus with borde rline MICs determined by MicroScan should be confirmed by an alternate method. The most practical and cost-effective means among those we te sted is the MRSA Screen Agar.