Vancomycin Minimum Inhibitory Concentration for Methicillin-Resistant Staphylococcus aureus Infections; Is There Difference in Mortality Between Patients?

BACKGROUND: New data indicates that vancomycin may be less effective against methicillin-resistant Staphylococcus aureus (MRSA) infections with minimum inhibition concentration (MIC) within a sensitive range. OBJECTIVES: The aim of this study was to determine the distribution of the vancomycin MIC between MRSA strains and observe the difference in mortality between patients, while the influence of changes in MIC on the efficacy of vancomycin was also examined. PATIENTS AND METHODS: A routine date-based study was conducted on 41 MRSA isolates in a hospital in Tehran, Iran. The isolates were assessed for MIC by using the E-test method, and results were categorized into three groups: A (MIC < 1.5 µg/mL), B (1.5 ≤ MIC < 2 µg/mL) and C (MIC ≥ 2 µg/mL) MRSA. RESULTS: Group A was the most common group, followed by groups C and B. Although there was no statistically significant difference between patients' mortality with the MIC group, the mortality rate of group A was higher than C and B. CONCLUSIONS: Regarding Clinical and Laboratory Standards Institute (CLSI) definition for vancomycin susceptibility (MIC < 2 µg/mL), it seems that vancomycin may not be considered as the best antibiotic in order to treat heteroresistant vancomycin intermediate S. aureus (hVISA) and vancomycin sensitive S. aureus (VSSA) infections, and a new breakpoint for vancomycin and alternative antibiotics should be considered.