Background: Significant discrepancies may exist between vancomycin minimum inhibitory concentrations (MICs) obtained from different testing methods among methicillin-resistant Staphylococcus aureus (MRSA) isolates. However, there is limited data regarding the clinical implications of these discrepancies. The objectives of this study were to determine if a significant difference exists between MRSA vancomycin MICs reported via the Etest and the VITEK 2 system and to determine if any differences observed have an impact on patient outcomes.
Methods: The study was a retrospective observational trial. The study population included patients admitted to Borgess Medical Center from November 1, 2012 through October 31, 2014. Any patient admitted with a blood culture positive for MRSA or patients in critical care units with a respiratory culture positive for MRSA were considered for inclusion. The MIC results from the VITEK 2 system and Etest were recorded. Patient outcome data collected included: in-hospital mortality, length of stay, duration of intubation and any anti-MRSA antibiotic changes as a result of the reported vancomycin MIC. The primary outcome was to determine if a significant difference existed between vancomycin MICs obtained via the Etest vs. VITEK 2 among MRSA isolates from blood and respiratory cultures.
Results: 119 patients were identified with a total of 331 blood and respiratory cultures positive for MRSA during the study period. A total of 140 cultures from 85 patients were included in the data analysis. See table 1 below for detailed results. The mean Etest MIC was significantly higher than the mean VITEK 2 MIC (2.09 mcg/mL vs. 0.93 mcg/mL, p<0.001). Given the large discrepancy between the MICs from each testing method, limited significant findings regarding patient outcomes were observed.
Table 1. MIC results for each testing method.
Conclusion: A significant discrepancy exists between the MRSA vancomycin MIC values reported via the Etest vs. the VITEK 2 system. The clinical implications of this discrepancy remain unclear and should be investigated further.
R. Maynard, None