Accuracy of Commercial Susceptibility Testing Method for Measuring Vancomycin MIC Against Methicillin-ResistantStaphylococcus aureus(MRSA)

Abstract

Methods: We evaluated the vancomycin (VAN) minimum inhibitory concentrations (MIC) on methicillin-resistant Staphylococcus aureus (MRSA) strains using MicroScan panel (Pos combo 3.1J). We also used the following 2 methods of preparing the bacterial solution: MicroScan Prompt system (P method) and manual preparation with inoculums adjusted to match a 0.5 McFarland turbidity standard (M method). The Clinical and Laboratory Standards Institute (CLSI) method was used as a reference. Results: On comparing the distribution of VAN MICs, the CLSI and M methods showed a good correlation, while the P method results were higher. Colony counts indicated the final inoculum concentrations from the P method were higher than those from the M method, which were within the ideal inoculum concentration. Comparing the results between the M method with the W/A 96 System (18hour incubation period) and the M method with a 24-hour incubation period revealed a discrepancy in the detection of some strains with 2.0 µg/mL of VAN. Conclusion: The P method appears to be less reliable than the M method for measuring VAN MIC against MRSA. Hence, the M method with a 24-hour incubation period should be used instead.