Gunnar Kahlmeter,d Arnfinn Sundsfjord,a,b on behalf of the NordicAST VRE Detection Study Group

Abstract

Different antimicrobial susceptibility testing methods to detect low-level vancomycin resistance in enterococci were evaluated in a Scandinavian multicenter study (n 28). A phenotypically and genotypically well-characterized diverse collection of Entero-coccus faecalis (n 12) and Enterococcus faecium (n 18) strains with and without nonsusceptibility to vancomycin was exam-ined blindly in Danish (n 5), Norwegian (n 13), and Swedish (n 10) laboratories using the EUCAST disk diffusion method (n 28) and the CLSI agar screen (n 18) or the Vitek 2 system (bioMérieux) (n 5). The EUCAST disk diffusion method (very major error [VME] rate, 7.0%; sensitivity, 0.93; major error [ME] rate, 2.4%; specificity, 0.98) and CLSI agar screen (VME rate, 6.6%; sensitivity, 0.93; ME rate, 5.6%; specificity, 0.94) performed significantly better (P 0.02) than the Vitek 2 system (VME rate, 13%; sensitivity, 0.87; ME rate, 0%; specificity, 1). The performance of the EUCAST disk diffusion method was challenged by differences in vancomycin inhibition zone sizes as well as the experience of the personnel in interpreting fuzzy zone edges as an indication of vancomycin resistance. Laboratories using Oxoid agar (P< 0.0001) or Merck Mueller-Hinton (MH) agar (

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