Comparison of Etest and agar dilution for determining minimum inhibitory concentration of vancomycin to healthcare-associated methicillin-resistant Staphylococcus aureus

Objectives: To compare agar dilution method and Etest in the determination of minimum inhibitory concentration (MIC) of vancomycin to healthcare-associated methicillin-resistant Staphylococcus aureus (HA-MRSA). Methods: A total of 98 non-duplicate strains of HA-MRSA isolated from different clinical specimens were tested for their antibiotic susceptibility pattern by Kirby-Bauer disk diffusion method and vancomycin MIC by agar dilution method and Etest (BioMerieux, France). Results: Out of 98 strains of HA-MRSA, 94 (95.9%) were vancomycin susceptible (MIC ≤2 μg/ml and 4 (4.1%) were vancomycin intermediate (MIC 4 μg/ml) by agar dilution method. By Etest, 53 (54.1%) were vancomycin susceptible, 4 (4.1%) were vancomycin intermediate, and the remaining 41 isolates had vancomycin MIC between 2 μg/ml and 4 μg/ml. Conclusion: Etest allows the detection of HA-MRSA strains with intermediate MIC values in addition to traditional dilutions. These properties will help in detection of MIC creep and also decision-making in using vancomycin for the treatment of serious infections caused by HA-MRSA.