Detection of colistin resistance via four methods among Gram-negative bacteria

Abstract

Objective: We aimed to find the most accurate and robust method for the detection of colistin resistance among Gram-negative bacteria by comparing the performance of four methods. Methods: Colistin resistantance was determine by disk diffusion (DD), Etest, colistin broth disk elution (CBDE) and compared with standard broth microdilution (BMD) method. Results: An exploratory study was conducted in a tertiary hospital in Jaipur, India, from November 2021 to November 2022. Of the 384 isolates tested, 41 (10.7%) were resistant against colistin by BMD and CBDE testing. Compared with BMD method, the categorical agreement of DD and Etest methods were 65.9% and 78.1%, respectively, and the rates of very major error were 34.1% and 21.9%, respectively. Conclusions: Due to the high frequency of very major error, the use of DD and Etest are not recommended by Clinical laboratory and standards institute (CLSI) for colistin susceptibility testing while CBDE is the recommended method which yields a categorical agreement of 100% with BMD method. The CBDE method can be used in routine laboratories for the detection of colistin resistance.