Rapid recognition of methicillin-resistant Staphylococcus aureus by use of automated test systems

The ability to rapidly recognize methicillin-resistant Staphylococcus aureus by use of two automated instrument systems, the MS-2 system (Abbott Laboratories, Diagnostics Division, Irving, Tex.) and the AutoMicrobic system (Vitek Systems, Hazelwood, Mo.), was evaluated on a collection of 95 methicillin-resistant S. aureus isolates recovered from at least six geographical areas of the United States. Isolates were simultaneously tested with both systems, and the results were compared with MIC tests performed by the National Committee for Clinical Laboratory Standards agar dilution method. Methicillin-resistant S. aureus isolates were defined as those with a methicillin MIC greater than or equal to 8 micrograms/ml by the reference procedure. Overall, with the AutoMicrobic system, 94.7% of 95 methicillin-resistant S. aureus isolates were detected, and with the MS-2 system, 91.6% of the isolates were detected. Isolates with methicillin MICs greater than or equal to 32 micrograms/ml were readily detected with both systems (41 of 42 isolates). Of 53 isolates from three locales with methicillin MICs of 8 or 16 micrograms/ml, 90.6% (48) were detected by the AutoMicrobic system, whereas 86.8% (46) were detected by the MS-2 system. A program update which has been added to the MS-2 system prints a warning message indicating possible methicillin-resistant S. aureus with isolates which demonstrate multiple antibiotic resistance (greater than or equal to four drugs other than methicillin). This warning message would have provided presumptive recognition of six of eight isolates with discrepant results for methicillin by the MS-2 system.