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. 1989 Sep;27(9):2140–2142. doi: 10.1128/jcm.27.9.2140-2142.1989

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

J M Swenson 1, B C Hill 1, C Thornsberry 1
PMCID: PMC267762  PMID: 2778080

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 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.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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