Problems with the disk diffusion test for detection of vancomycin resistance in enterococci

Abstract

A total of 53 strains of enterococci, including recently isolated strains with high-level resistance to vancomycin, were tested for vancomycin susceptibility by broth microdilution and disk diffusion using Mueller-Hinton media with and without supplementation with 5% blood. By using currently published parameters of the National Committee for Clinical Laboratory Standards for the disk diffusion test, we found that strains for which MICs were 8 to 32 μg/ml were incorrectly placed in the susceptible or intermediate category, which caused both very major (1.9%) and minor (11.5%) errors. When we used newer, recently proposed breakpoints for vancomycin, we found 13.5% minor errors but no very major errors. Changing disk diffusion breakpoints to ≤ 14 mm for susceptible and ≥ 15 mm for resistant would eliminate the problem for the strains with MICs of 32 μg/ml but not for those with MICs of 8 μg/l. For those strains, it is necessary to perform an MIC test to differentiate them from strains with MICs of ≤ 4 μg/ml.