Objective: The aims of this study were to evaluate the performance of the Modified Broth Disk Elution (BDE) test with diminished test tubes and colistin disks, compare it with the reference method and consequently hope to present an alternative method for busy microbiology laboratories.

Method: A total of 100 colistin-susceptible and 50 colistin-resistant gram-negative bacteria, isolated from various clinical samples in our hospital between 2020-2022 with the automated system were included in this study. The results of the modified BDE method and the automated system were compared with the reference method as broth microdilution (BMD), with categorical agreement (CA), very major error (VME) and major error (ME) rates.

Results: Overall, the categorical agreement (CA) rate of the automated system was 86%, VME rate was 10.7%, and ME rate was 3.3%. In the modified BDE method, these rates were 80%, 4% and 16%, respectively. Ten of the 16 (62.5%) isolates which were interpreted as VME in the automated system, were found to be resistant in modified BDE method. Similarly, three of five (60%) isolates which were interpreted as ME, were found to be susceptible in the modified BDE method.

Conclusion: Although the CA of the automated system is close to ISO standards, it was found that the rate of VME was very high in terms of colistin sensitivity, and this rate decreased when these were studied with the modified BDE method. When colistin susceptibility tests could not be studied with the BMD method, and if isolates of Enterobacteriaceae and Pseudomonas spp. are found as susceptible in the automated system, the modified BDE method gives more reliable results.