997. Vancomycin MIC Differences with Etest vs. VITEK 2 Among Methicillin-Resistant Staphylococcus aureus Isolates at a Tertiary Care Center
Session: Poster Abstract Session: Diagnostic Microbiology: Staphylococci
Friday, October 9, 2015
Room: Poster Hall
  • ID Week Poster Final Draft.pdf (392.8 kB)
  • 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.


    Mitchell Stein, Pharm.D., Justin Rak, Pharm.D. and Rebecca Maynard, Pharm.D., Pharmacy, Borgess Medical Center, Kalamazoo, MI


    M. Stein, None

    J. Rak, None

    R. Maynard, None

    Previous Abstract | Next Abstract >>

    Findings in the abstracts are embargoed until 12:01 a.m. PDT, Wednesday Oct. 7th with the exception of research findings presented at the IDWeek press conferences.