Affiliation:
1. Department of Pediatrics
2. Department of Pathology & Immunology, Washington University School of Medicine, St. Louis, Missouri 63110
3. Barnes-Jewish Hospital, St. Louis, Missouri 63110
Abstract
ABSTRACT
In January 2006, the Clinical and Laboratory Standards Institute (CLSI) updated breakpoints for vancomycin susceptibility testing for
Staphylococcus aureus
such that an MIC greater than 2 mg/liter was considered to represent nonsusceptibility to vancomycin. Previously, an MIC of 4 mg/liter had been considered to represent susceptibility. Additionally, in 2009, the CLSI altered the guidelines for staphylococci such that disk diffusion was no longer an acceptable means for testing vancomycin susceptibility in these organisms. To accommodate the change in breakpoints and methodological updates, we designed a medium consisting of brain heart infusion agar with 3 mg/liter vancomycin (BHI-V3) to screen for isolates of
S. aureus
with reduced susceptibility to vancomycin. We challenged this medium using a previously characterized collection of 100 isolates of
S. aureus
, including 55 vancomycin-susceptible isolates and 45 isolates representing vancomycin-intermediate strains of
S. aureus
(VISA) (with vancomycin MICs of 4 mg/liter or greater). All of the VISA isolates grew on the agar, for 100% sensitivity. Nineteen vancomycin-susceptible isolates also grew on the agar, for 65% specificity. We then incorporated BHI-V3 into clinical practice. In the first 60 days postimplementation, we identified 17 potential VISA isolates out of 421
S. aureus
isolates tested. Thirteen out of the 17 were confirmed to represent VISA isolates. In light of the excellent sensitivity of this medium, we recommend that clinical laboratories incorporate BHI-V3 to screen for vancomycin-nonsusceptible isolates of
S. aureus
.
Publisher
American Society for Microbiology
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at Hershey Medical Center
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