Multicenter evaluation of colistin broth disk elution and colistin agar test: A report from the clinical and laboratory standards institute

Romney M. Humphries*, Daniel A. Green, Audrey N. Schuetz, Yehudit Bergman, Shawna Lewis, Rebecca Yee, Stephania Stump, Mabel Lopez, Nenad MacEsic, Anne Catrin Uhlemann, Peggy Kohner, Nicolynn Cole, Patricia J. Simner

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

59 Scopus citations


Susceptibility testing of the polymyxins (colistin and polymyxin B) is challenging for clinical laboratories. The Clinical and Laboratory Standards Institute (CLSI) Antimicrobial Susceptibility Testing Subcommittee evaluated two methods to enable accurate testing of these agents. These methods were a colistin broth disk elution (CBDE) and a colistin agar test (CAT), the latter of which was evaluated using two inoculum volumes, 1 μl (CAT-1) and 10 μl (CAT-10). The methods were evaluated using a collection of 270 isolates of Enterobacterales, 122 Pseudomonas aeruginosa isolates, and 106 Acinetobacter spp. isolates. Overall, 94.4% of CBDE results were in essential agreement and 97.9% in categorical agreement (CA) with reference broth microdilution MICs. Nine very major errors (VME; 3.2%) and 3 major errors (ME; 0.9%) were observed. With the CBDE, 98.6% CA was observed for Enterobacterales (2.5% VME, 0% ME), 99.3% CA was observed for P. aeruginosa (0% VME, 0.7% ME), and 93.1% CA was observed for Acinetobacter spp. (5.6% VME, 3.3% ME). Overall, CA was 94.9% with 6.8% VME using CAT-1 and improved to 98.3% with 3.9% VME using CAT-10. No ME were observed using either CAT-1 or CAT-10. Using the CAT-1/CAT-10, the CA observed was 99.4%/99.7% for Enterobacterales (1%/0.5% VME), 98.7%/100% for P. aeruginosa (8.3%/0% VME), and 88.5%/92.3% for Acinetobacter spp. (21.4%/14.3% VME). Based on these data, the CLSI antimicrobial susceptibility testing (AST) subcommittee endorsed the CBDE and CAT-10 methods for colistin testing of Enterobacterales and P. aeruginosa.

Original languageAmerican English
Article numbere01269-19
JournalJournal of Clinical Microbiology
Issue number11
StatePublished - 2019
Externally publishedYes

Bibliographical note

Publisher Copyright:
Copyright © 2019 American Society for Microbiology. All Rights Reserved.


  • Acinetobacter
  • Agar dilution
  • Broth microdilution
  • Colistin
  • Colistin broth disk elution
  • Enterobacterales
  • Pseudomonas aeruginosa
  • Susceptibility testing


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