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Journal of Clinical Microbiology, July 2009, p. 2013-2017, Vol. 47, No. 7
0095-1137/09/$08.00+0     doi:10.1128/JCM.00221-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Accuracy of Commercial and Reference Susceptibility Testing Methods for Detecting Vancomycin-Intermediate Staphylococcus aureus{triangledown}

Jana M. Swenson,* Karen F. Anderson, David R. Lonsway, Angela Thompson, Sigrid K. McAllister, Brandi M. Limbago, Roberta B. Carey, Fred C. Tenover,{dagger} and Jean B. Patel

Clinical and Environmental Microbiology Branch, Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia 30333

Received 2 February 2009/ Returned for modification 12 March 2009/ Accepted 29 April 2009

We compared the results obtained with six commercial MIC test systems (Etest, MicroScan, Phoenix, Sensititre, Vitek Legacy, and Vitek 2 systems) and three reference methods (agar dilution, disk diffusion, and vancomycin [VA] agar screen [VScr]) with the results obtained by the Clinical and Laboratory Standards Institute broth microdilution (BMD) reference method for the detection of VA-intermediate Staphylococcus aureus (VISA). A total of 129 S. aureus isolates (VA MICs by previous BMD tests, ≤1 µg/ml [n = 60 strains], 2 µg/ml [n = 24], 4 µg/ml [n = 36], or 8 µg/ml [n = 9]) were selected from the Centers for Disease Control and Prevention strain collection. The results of BMD with Difco Mueller-Hinton broth were used as the standard for data analysis. Essential agreement (percent ±1 dilution) ranged from 98 to 100% for all methods except the method with the Vitek Legacy system, for which it was 90.6%. Of the six commercial MIC systems tested, the Sensititre, Vitek Legacy, and Vitek 2 systems tended to categorize VISA strains as susceptible (i.e., they undercalled resistance); the MicroScan and Phoenix systems and Etest tended to categorize susceptible strains as VISA; and the Vitek Legacy system tended to categorize VISA strains as resistant (i.e., it overcalled resistance). Disk diffusion categorized all VISA strains as susceptible. No susceptible strains (MICs ≤ 2 µg/ml) grew on the VScr, but all strains for which the VA MICs were 8 µg/ml grew on the VScr. Only 12 (33.3%) strains for which the VA MICs were 4 µg/ml grew on VScr. The differentiation of isolates for which the VA MICs were 2 or 4 µg/ml was difficult for most systems and methods, including the reference methods.

* Corresponding author. Mailing address: Centers for Disease Control and Prevention, Mailstop G08, 1600 Clifton Road, Atlanta, GA 30333. Phone: (404) 639-0196. Fax: (404) 638-1381. E-mail: jswenson{at}cdc.gov

{triangledown} Published ahead of print on 6 April 2009.

{dagger} Present address: Cepheid, 904 Caribbean Drive, Sunnyvale, CA 94089.

Journal of Clinical Microbiology, July 2009, p. 2013-2017, Vol. 47, No. 7
0095-1137/09/$08.00+0     doi:10.1128/JCM.00221-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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