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J Clin Microbiol. 2019 Jan 30;57(2). pii: e01163-18. doi: 10.1128/JCM.01163-18. Print 2019 Feb.

Two-Site Evaluation of the Colistin Broth Disk Elution Test To Determine Colistin In Vitro Activity against Gram-Negative Bacilli.

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Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA
Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
Department of Pathology and Laboratory Medicine, UCLA Health, Los Angeles, California, USA.
Accelerate Diagnostics, Tucson, Arizona, USA.
Department of Pediatrics, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
Department of Pathology and Laboratory Medicine, University of Arizona, Tuscon, Arizona, USA.


Limited methods for colistin MIC determination are available to clinical microbiology laboratories. The purpose of this study was to evaluate the accuracy of the colistin broth disk elution (CBDE) test compared to that of broth microdilution (BMD) for identifying colistin MICs. CBDE was compared to colistin BMD using a collection of Gram-negative bacilli tested at two U.S. microbiology laboratories. The isolates tested included 121 retrospective clinical isolates, 45 prospective clinical isolates, and 6 mcr-1-positive Escherichia coli isolates. CBDE was performed with four 10-ml cation-adjusted Mueller-Hinton broth tubes per isolate, to which 0, 1, 2, and 4 colistin 10-µg disks were added, generating final concentrations in the tubes of 0 (growth control), 1, 2, and 4 µg/ml, respectively. MICs were evaluated visually and interpreted using Clinical and Laboratory Standards Institute breakpoints. Site 2 also compared CBDE to the reference broth macrodilution (BMAD) method (n = 110 isolates). Overall, CBDE yielded a categorical agreement (CA) and essential agreement (EA) of 98% and 99%, respectively, compared to the results of colistin BMD. Very major errors occurred for mcr-1-producing strains, with MICs fluctuating from 2 to 4 µg/ml on repeat testing. The results for all other isolates were in CA with those of BMD. CBDE versus BMAD had an EA of 100% and a CA of 100%. Compared to currently used techniques, CBDE is an easy and practical method to perform colistin MIC testing. Some mcr-1-producing isolates yielded MICs of 2 µg/ml by CBDE and 4 µg/ml by BMD. As such, the results for isolates with colistin MICs of 2 µg/ml by CBDE should be confirmed by the reference BMD method, and isolates with MICs of ≥2 µg/ml should be evaluated for the presence of mcr genes.


antimicrobial susceptibility testing; colistin; colistin broth disk elution; method

[Available on 2019-07-30]

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