Investigation of Colistin Susceptibility in Carbapenem-Resistant Gram-Negative Bacteria

Objective: We evaluated colistin susceptibility in carbapenem-resistant isolates and compared the automated microbiology system (VITEK 2 Compact, bioMérieux, France) with the broth microdilution (BMD) reference method.Materials and Methods: A total of 174 carbapenem-resistant Gram-negative isolates collected from various clinical specimens during 2022–2023 were included in the study. Identification and antimicrobial susceptibility testing were performed using conventional methods and the VITEK 2 Compact automated system (bioMérieux, France). Colistin susceptibility was confirmed by the BMD reference method.Results: Among 90 Enterobacterales isolates tested with BMD, 39 (43.3%) were susceptible and 51 (56.7%) were resistant. In the non-fermentative group, 67 (79.8%) of 84 isolates were susceptible, and 17 (20.2%) were resistant. When compared with BMD, VITEK 2 demonstrated an essential agreement (EA) of 68.8%, a categorical agreement (CA) of 80%, a very major error (VME) rate of 7.8%, and a major error (ME) rate of 35.9% for Enterobacterales. For non-fermentative isolates, EA and CA were markedly lower (54.7% and 73.8%, respectively), while VME (47%) and ME (20.9%) rates exceeded acceptable ISO performance thresholds.Conclusion: Although automated systems such as VITEK 2 streamline laboratory workflow, their colistin susceptibility results show poor concordance with the BMD reference method, particularly for non-fermentative isolates. The high error rates observed in this study highlight the critical importance of confirming automated colistin results with BMD to avoid misclassification and inappropriate antimicrobial therapy.