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

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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 languageEnglish
Article numbere01269-19
Number of pages10
JournalJournal of Clinical Microbiology
Issue number11
Publication statusPublished - 1 Nov 2019
Externally publishedYes


  • Acinetobacter
  • agar dilution
  • broth microdilution
  • colistin
  • colistin broth disk elution
  • Enterobacterales
  • Pseudomonas aeruginosa
  • susceptibility testing

Cite this

Humphries, R. M., Green, D. A., Schuetz, A. N., Bergman, Y., Lewis, S., Yee, R., Stump, S., Lopez, M., Macesic, N., Uhlemann, A-C., Kohner, P., Cole, N., & Simner, P. J. (2019). Multicenter Evaluation of Colistin Broth Disk Elution and Colistin Agar Test: a Report from the Clinical and Laboratory Standards Institute. Journal of Clinical Microbiology, 57(11), [e01269-19].