Comparison of MIC COL, Sensititre™ and Colistin Agar Screening Plate Methods with Broth Microdilution Reference Method for Detection of Colistin Susceptibility in Carbapenem-Resistant Gram-negative Bacteria

Objective: Colistin is one of the last choice antibiotics used to treat infections caused by carbapenem-resistant Gram-negative bacteria. Accurate detection of colistin sensitivity is important in the treatment of these infections. The reference method for colistin susceptibility testing is broth microdilution, but its use in routine laboratories is limited due to its laborious process. Therefore, an easy-to-use and accurate test is needed. This study aimed to compare the performance of two commercial tests (Sensititre (TM) and MIC COL) based on broth microdilution method (BMD) and colistin agar screening plate methods with reference broth microdilution and investigate their suitability for use in routine practice. Methods: In this study, the performance of the methods was evaluated with carbapenem-resistant Klebsiella spp. (n=93), Acinetobacter baumannii-calcoaceticus complex (n=83) and Pseudomonas spp. (n=42) isolates. Results: Of the isolates, 42 (19.3%) [K. pneumoniae (n=26), Acinetobacter baumannii-calcoaceticus complex (n=9), P. aeruginosa (n=6), K. aerogenes (n=1)] were determined to be colistin-resistant with reference broth microdilution method. The categorical agreement rates of the Sensititre (TM) (Thermo Fisher Scientific, Cleveland, USA), MIC COL (Diagnostics Inc., Galanta, Slovakia), and colistin screening plate methods with reference broth microdilution method were 94.9%, 96.3%, and 95.9% / 93.6% (24/48 hours), respectively. The essential agreement rates of the two commercial broth microdilution tests were found to be over 87% for all isolates. The agreement rates were found to be lower in all methods tested with Pseudomonas spp. isolates. Very major error rates were found to be high in Acinetobacter baumannii-calcoaceticus complex, and P. aeruginosa isolates. Conclusions: BMD-based commercial systems and colistin agar screening plate, which do not require additional equipment and experience, can be used routinely in Klebsiella spp. isolates; but results obtained with these methods should be confirmed with reference broth microdilution in nonfermantative bacilli.