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No. 57 (PM 6)
Differences of Vancomycin MICs for MRSA Isolates Based upon the study
Susceptibility Test Method Used
K Watanabe, M Mikami, Y Saya, C Tanaka, F Omata, K Furukawa, K Takeda
St. Luke’s International Hospital, Japan
Background: Recent studies reported increasing trend of vancomycin (VCM) treatment failure in
methicillin-resistant Staphylococcus aureus (MRSA) blood stream infections. Differences in VCM MIC
values among different tests are also reported. The aim of this study was to evaluate recent change in the
prevalence of MRSA with high VCM MIC(=2.0) and to compare the results of the three methods such
as E-test®, VTEK2® and Broth microdilution (BMD). Methods: E-test® ,VITEK2® and BMD were
applied for measuring VCM MIC in consecutive 59 MRSA strains isolated from blood cultures of 59
patients between 2007 and 2012. The tests results were categorized to binary variables using cut-off
value of 2.00μg/ml to calculate the kappa statistic. The annual change of prevalence of VCM resistant
MRSA was tested by Fisher’s exact test.
Results: MRSA were isolated from blood cultures in 28 cases in 2008,12 cases in 2009, 9 cases in 2010,
and 10 cases in 2011. The prevalence of strain with VCM MIC 2.0μg/ml using BMD method was 14%
in 2008, 28.6% in 2009, 0% in 2010, and 10% in 2011. There was no significant annual change in the
prevalence of VCM resistant MRSA (P=0.23 by Fisher’s exact test).The proportion of strain with MIC
2.00μg/ml of BMD, VITEK2® and E-test® method was 15.2%, 0.5% and 59%, respectively. The kappa
statistic of three tests was 0.12. Conclusion: There is no significant change in the prevalence of MRSA
with high VCM MIC (=2.0) measured by BMD method from 2008 to 2011. Comparing to BMD method,
VITEK2® method tends to miss MRSA with high VCM MIC (=2.0). On the other hand, E-test®
method tends to overdiagnose MRSA with high VCM MIC (=2.0). The differences between tests’
characteristics should be taken into account for clinical implication
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