Multi-center evaluation of colistin broth disk elution and colistin agar test: a report from the Clinical and Laboratory Standards Institute

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 and 106 Acinetobacter spp. 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%) were observed and 3 major errors (ME, 0.9%). By the CBDE, 98.6% CA was observed for Enterobacterales (2.5% VME, 0% ME), 99.3% CA for P. aeruginosa (0% VME, 0.7% ME) and 93.1% CA for Acinetobacter spp. (5.6% VME, 3.3% ME). Overall, CA was 94.9% with 6.8% VME by CAT-1 and improved to 98.3% with 3.9% VME by CAT-10. No ME were observed by either CAT-1 or CAT-10. By the CAT-1/CAT-10, 99.4%/99.7% CA was observed for Enterobacterales (1%/0.5% VME), 98.7%/100% CA for P. aeruginosa (8.3%/0% VME) and 88.5%/92.3% CA for Acinetobacter spp. (21.4%/14.3% VME). Based on these data, the CLSI AST Subcommittee endorsed the CBDE and CAT-10 methods for colistin testing of the Enterobacterales and P. aeruginosa.