Differences in vancomycin MIC among MRSA isolates by agar dilution and E test method

In this study, the correlation between vancomycin minimum inhibitory concentration (MIC) obtained by the E test technique and the Clinical And Laboratory Standards Institute (CLSI) agar dilution method was evaluated. A total of 53 Methicillin Resistant Staphylococcus aureus (MRSA) strains were tested by both the methods in the present study. MICs of vancomycin obtained by the E test method were consistently higher (+0.5 to 2 log2 dilutions) than those obtained by the agar dilution method. Out of 53 MRSA isolates, 49 isolates showed higher MIC results by E test than by agar dilution method. Three isolates showed same MIC result by both methods. Since many studies have demonstrated increased clinical failure with MRSA isolates for which vancomycin MICs are increased (>1 μg/ml) but still within the susceptibility range (≤ 2 μg/ml), our findings suggest the requirement to re-look into the breakpoints for vancomycin for determining sensitivity of MRSA isolates. Guidelines should also specify the method to be used for determining the MIC.