Evaluation of the Etest GRD for the detection of Staphylococcus aureus with reduced susceptibility to glycopeptides

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. 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. 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. Etest GRD was simple to perform and may be feasible for clinical microbiology laboratories. This test may be useful for clinical detection of hVISA.