Problems with the disk diffusion test for detection of vancomycin resistance in enterococci.
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 micrograms/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 less than or equal to 14 mm for resistant [corrected] and greater than or equal to 15 mm for susceptible [corrected] would eliminate the problem for the strains with MICs of 32 micrograms/ml but not for those with MICs of 8 micrograms/ml. For those strains, it is necessary to perform an MIC test to differentiate them from strains with MICs of less than or equal to 4 micrograms/ml.
Swenson
J M
JM
Antimicrobics Investigation Branch, Centers for Disease Control, Atlanta, Georgia 30333.
Hill
B C
BC
Thornsberry
C
C
D004352:Drug Resistance, Microbial; D006801:Humans; D008826:Microbial Sensitivity Tests; D013291:Streptococcus / Q000187:drug effects*; D014640:Vancomycin / Q000494:pharmacology*
D014640:Vancomycin