JAC Advance Access published online on January 9, 2009
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkn520
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Original research |
Evaluation of the Etest GRD for the detection of Staphylococcus aureus with reduced susceptibility to glycopeptides
1 Anti-Infective Research Laboratory, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, 259 Mack Ave., Detroit, MI 48201, USA 2 Detroit Receiving Hospital, 4201 Saint Antoine St., Detroit, MI 48201, USA 3 Northeastern University, School of Pharmacy, 206 Mugar Building, 360 Huntington Ave., Boston, MA 02115, USA 4 School of Medicine, Wayne State University, Detroit, MI 48201, USA
Received 18 September 2008; returned 21 October 2008; revised 17 November 2008; accepted 2 December 2008
* Corresponding author. Anti-Infective Research Laboratory, Pharmacy Practice—4148, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, 259 Mack Ave., Detroit, MI 48201, USA. Tel: +1-313-577-4376; Fax: +1-313-577-8915; E-mail: m.rybak{at}wayne.edu
Objectives: Continued glycopeptide-selective pressure has led to non-susceptible strains of Staphylococcus aureus including heterogeneously vancomycin-intermediate S. aureus (hVISA). The gold standard for identification of hVISA is the population analysis profile area under the curve ratio (PAP-AUC), though this method is time-consuming and labour-intensive. The objective of this study was to compare a new method for detection of hVISA, the Etest GRD, to PAP-AUC and to macro Etest.
Methods: One hundred clinical hVISA and 50 clinical fully vancomycin-susceptible S. aureus (VSSA), confirmed by PAP-AUC, were evaluated. Microtitre and Etest MICs were determined by standard testing procedures on all isolates. Macro Etest was performed according to referenced procedures. The Etest GRD was tested using a 0.5 McFarland standard on Mueller–Hinton agar + 5% blood and read at 24 and 48 h. If either the vancomycin or the teicoplanin end of the GRD strip was 
8 and the vancomycin Etest was 
4, the isolate was considered hVISA.
Results: Vancomycin MIC50/MIC90 for hVISA and VSSA was 1.5/2 mg/L and 1/1.5 mg/L, respectively, by Etest and vancomycin MIC50/MIC90 for hVISA and VSSA was 1/2 mg/L for both by microtitre; MIC values for hVISA being significantly higher (P 0.023). At 24 h, the Etest GRD was 77% sensitive and 98% specific, and at 48 h, it was 93% sensitive and 82% specific compared with PAP-AUC. Macro Etest was 83% sensitive and 94% specific at 48 h.
Conclusions: Etest GRD was simple to perform and may be feasible for clinical microbiology laboratories. This test may be useful for clinical detection of hVISA.
Key Words: hVISA , hGISA , heteroresistance , MRSA
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