Detection of decreased susceptibility of Staphylococcus aureus to vancomycin with the VITEK 2 System

Abstract number: p453

Engelhard  K., Griffith  R., Mayer  J., Slaughter  J., Ullery  M., Beiner  L., Messina-Powell  S., Pyse  D., Zambardi  G., Shortridge  D.


The purpose of this study was to determine if the combination of the current vancomycin VITEK 2 MIC test and a new vancomycin resistant Staphylococcus aureus (VRSA) screen test* could accurately detect strains of S. aureus with decreased susceptibility to vancomycin. Recently, four VRSA strains were detected in the United States. These strains were not reliably detected with the current vancomycin MIC test on VITEK 2. Due to the acquisition of vanA, these VRSA strains grow very differently than the vancomycin intermediate S. aureus (VISA) or the hetero-resistant S. aureus (hVISA). A new VRSA screen test for S. aureus was developed to specifically flag VRSA strains. This screen test uses a different medium formulation to increase expression of resistance in these VRSA strains.


Seventy-two S. aureus isolates (including strains received from CDC and NARSA) with vancomcyin MICs of 2–8 mg/ml were tested on VITEK 2 AST-P541 cards. Reference MICs were determined by broth microdilution according to Clinical Laboratory Standards Institute (CLSI) guidelines. VITEK 2 vancomycin MIC results, along with corresponding Advanced Expert System (AES) findings were evaluated. The current CLSI breakpoints for vancomycin are <=4 S, 8–16 I, >=32 R (with a note stating that any strain with a result of >=4 be sent to a reference laboratory).


The essential agreement was 95.8% (69/72), category agreement was 90.3% (65/72), with 9.7% (7/72) minor errors, and no major errors. Six of the seven minor errors were one-dilution errors with a reference MIC of 8 mg/ml calling 4 mg/ml by VITEK 2. Each of the six strains was flagged by AES as a VISA phenotype. Only the known VRSA strains gave a positive VRSA screen result. Susceptible, VISA, and hVISA strains gave negative VRSA screen results. A positive screen test strongly suggests that a VRSA may be present, however, resistance to vancomycin must be confirmed by performing an offline test as recommended by CLSI (M100-S15, vol. 25 no.1, January 2005) or as recommended by the local authorities.


These data indicate that the VITEK 2 can accurately determine decreased susceptibility to vancomycin by using the combination of the current MIC test (for susceptible, VISA, and hVISA strains) and the VRSA screen test (for VRSA). *The new VRSA screen test for S. aureus has not been cleared for use with the VITEK 2 system by the United States FDA and is not yet available for commercial use.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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