Validation of Vitek version 7.01 software for testing staphylococci against vancomycin

We tested 143 isolates of staphylococci with vancomycin by the National Committee for Clinical Laboratory Standards broth microdilution (BMD) reference method and compared the results to those generated using the Vitek automated system (GPS-105 and GPS-107 cards and version 7.01 software). For ten isolates, the vancomycin MICs by BMD were 8 microg/ml. By Vitek, the vancomycin MICs ranged from 2 to 16 microg/ml. Vancomycin MICs of > or =32 microg/ml were reported for two additional isolates by Vitek; however, the MICs decreased to < or =0.5 microg/ml on retesting. By BMD, the vancomycin MICs for both isolates were 1 microg/ml. While the modal vancomycin MIC results by BMD for S. aureus and coagulase-negative staphylococci (CoNS) were both 1 microg/ml, Vitek results showed a mode of < or =0.5 microg/ml for S. aureus, and a mode of 2 microg/ml for CoNS. Vitek did not report vancomycin MICs of 1 or 4 microg/ml for any of the isolates tested. While the sensitivity of detecting staphylococci with reduced susceptibility to vancomycin appears to be improved with Vitek version 7.01 software, when compared to earlier software versions, laboratories may notice an overall shift in MIC data toward higher vancomycin MICs, although for the most part, this does not affect the categorical interpretations of the results.