Comparison of Broth Disk Elution and Broth Microdilution Methods for Colistin Susceptibility Testing in Carbapenem-Resistant Enterobacteriaceae and Pseudomonas aeruginosa.

BACKGROUND: The increasing prevalence of carbapenem-resistant Enterobacteriaceae (CRE) and Pseudomonas aeruginosa (CRPA) poses a significant challenge to clinical management, particularly in resource-limited settings. Colistin remains one of the last-resort antibiotics for treating these infections; however, its accurate susceptibility testing remains technically challenging. The Broth Microdilution (BMD) method, though recommended as the reference standard, is labor-intensive and not feasible for routine use in most laboratories. This study aims to evaluate the Colistin Broth Disk Elution (CBDE) method as a practical alternative to BMD. MATERIALS AND METHODS: This cross-sectional study was conducted over 18 months (December 2022 to June 2024) at a tertiary care microbiology laboratory in India. A total of 120 non-duplicate CRE and CRPA isolates from clinical specimens were randomly selected and tested for colistin susceptibility using both BMD and CBDE methods. MIC interpretations were based on CLSI 2024 guidelines. Categorical agreement (CA), essential agreement (EA), and error rates were analyzed as per CLSI 2015 recommendations. RESULTS: Out of 120 carbapenem-resistant isolates included in the study, 92 (76.67%) were CRE and 28 (23.33%) were CRPA. Colistin resistance was detected in six (5%) and four (3.33%) isolates by BMD and CBDE, respectively. CBDE showed a CA of 98.3% (95% confidence Interval: 94.1%-99.8%) and EA of 100% when compared to BMD, with only two minor errors (1.6%) and no major or very major errors. CONCLUSION: CBDE demonstrated high categorical agreement with BMD and minimal error and offers a simple, cost-effective alternative for routine colistin susceptibility testing, especially in low-resource settings where BMD is not feasible.