A comparison of BHI/vancomycin agar, VITEK, and Kirby-Bauer methodologies for identifying Vancomycin resistance in enterococcal isolates from laboratory specimens
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A comparison of BHllvancomycin agar. VITEK. and Kirby-Bauer disk. diffusion methodologies for identifying vancomycin resistance in Enterococcal isolates was conducted. One hundred-six Enterococcal isolates were collected from several clinical sites in the Northern Illinois region and tested to determine vancomycin susceptibility by each of the three methodologies. Of the 106 isolates tested. 23 were determined by the BHllvancomycin agar method to be vancomycin resistant and 83 were determined to be vancomycin susceptible. VITEK correctly identified all of the 83 vancomycin susceptible isolates (100%) and 20 of the 23 vancomycin resistant isolates (87%). Kirby-Bauer disk. diffusion cooectly identified all of the 83 vancomycin susceptible isolates (100%) and 22 of the 23 vancomycin resistant isolates (95.6%). The sensitivity and specificity of each of the three methodologies evaluated in this study are quite high; however. whether or not these values justify using only VITEK or Kirby-Bauer susceptibility testing without a confirmatory method can not be determined by this rather limited study alone. These results do confirm that incorrect susceptibility testing results occur with both VITEK and Kirby-Bauer methodologies, particularly in determining vancomycin resistance. Until additional studies confirm that automated MIC determination Q" Kirby-Bauer susceptibility testing consistently give reliable results. confirmatory testing with BHllvancomycin agar plates will ensure that accurate vancomycin susceptibility results are reported for Enterococcal isolates.