Comparison of colistin drop test to broth micro-dilution for the screening of colistin resistance in carbapenem-resistant gram-negative organisms
DOI:
https://doi.org/10.61529/idjp.v35i1.369Abstract
Background: Colistin is a last-resort agent for treating infections caused by carbapenem-resistant gram-negative bacteria. However, standard susceptibility testing methods are complex and resource-intensive. This study aimed to evaluate the agreement of the colistin drop test with the reference standard for detecting colistin resistance.
Material and Methods: This prospective observational study was conducted from 1st January 2022 to 31st December 2023 at the Aga Khan University Hospital clinical laboratory, Karachi. The colistin drop test was evaluated using three solutions: one prepared from colistin powder and two from elution of colistin and polymyxin B disks. Carbapenem-resistant Escherichia coli, Enterobacter spp., Klebsiella spp., and Pseudomonas aeruginosa isolated from clinical samples (January–December 2022) were included. All methods were compared with breakpoint BMD; discrepant isolates were confirmed by reference BMD and categorized as intermediate or resistant according to CLSI guidelines.
Results: Out of 190 isolates, 85 (44.8%) were colistin-resistant and 105 (55.2%) intermediates by breakpoint BMD. Categorical agreement for colistin powder, colistin disk, and polymyxin disk methods was 97%, 95%, and 98%, respectively. Corresponding positive predictive values were 100%, 95%, and 100%; negative predictive values 96%, 96%, and 97%; sensitivity 95%, 96%, and 96%; and specificity 100%, 96%, and 100%.
Conclusion: Study findings align with previous research. The colistin drop test using polymyxin B disk solution showed optimal performance and offers a simple, reliable method for detecting colistin resistance in Gram-negative organisms in resource-limited settings.
Keywords: Carbapenem-resistant Enterobacterales, Carbapenem-resistant Pseudomonas aeruginosa, Colistin drop test (CT-DT), Colistin resistance screening, Colistin susceptibility testing
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Copyright (c) 2026 Rija Zehra, Imran Ahmed, Mohammad Umer Khan, Tooba Raheem, Syed Abbas Kazmi, Sadaf Zaka, Kauser Jabeen
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