Background and study aims: Antimicrobial susceptibility testing (AST) for colistin is challenging for clinical laboratories, and its use without prior testing has fostered drug resistance. The CLSI recommended colistin broth disk elution (CBDE) and colistin agar test (CAT) for testing colistin. The current study aimed to evaluate the accuracy of the CBDE test and CAT compared to the reference broth microdilution (BMD) to determine colistin MICs.
Materials and methods: This cross-sectional study was performed on 62 MDR Enterobacteriaceae isolates collected from various clinical samples submitted for routine culture and sensitivity in the Main Microbiology Laboratory, Ain Shams University Hospitals, from July 2022 to January 2023.
Results: For CBDE, the categorical agreement (CA) was 98.4%, the Essential agreement (EA) was 95.1%, one Very major error was observed (VME) 2.7%, and no Major error (ME) 0%. For CAT, a CA of 95.2%, an EA of 88.7%, two VMEs of 5.4% and one ME of 4% were observed.
Conclusion: The CBDE is a precise, uncomplicated, and practical test for identifying colistin MICs. It showed high CA and EA with low VME, rendering it a reliable method for colistin susceptibility testing. The CAT is handy and can be implemented as a part of routine AST of colistin. CAT is relatively easier to execute than CBDE because up to ten isolates can be inoculated per dilution plate. Our study showed a VME of 5.4% and a Major error of 4% for CAT, requiring further evaluation and studies for this test.