Comparison of six different methods used in detection of colistin resistance in extensively drug-resistant K. pneumoniae, A. baumannii and P. aeruginosa isolates with the reference method broth microdilution method.

Objective: This study aimed to evaluate the agreement of different methods with the reference method in the detection of colistin resistance in extensively drug-resistant K. pneumoniae, A. baumannii and P. aeruginosa isolates.

Materials and methods: K. pneumoniae, A. baumannii and P. aeruginosa isolates were included in the study. All isolates were subjected to colistin susceptibility testing with seven different methods. All results were compared with broth microdilution results. Categorical agreement (CA), very majorl error (VME) and major error (ME) rates were calculated for the tested methods.

Results: In all methods, VME and ME rates were found to be above the acceptable performance limit for all strains. Although VME rates were above the target value in all strains, the lowest VME rates were detected in MRRP-NP with 12.5%. Among all isolates, the lowest ME rate was obtained in MCLDE with 10% and the highest ME rate was detected in VITEK 2^® with 19.9%. The highest ME rate was detected in VITEK 2^® with 27.3%. The highest CA in A. baumannii was detected in MRRP-NP; in P. aeruginosa, it was detected in MCLDE, MCLDER and MRRP-NP. CA in K. pneumoniae isolates was below the acceptable performance in all methods.

Conclusions: The results obtained in this study show that the methods used in colistin susceptibility testing still need standardization. There is an urgent need for a colistin susceptibility testing method that is easy to apply, inexpensive and suitable for routine use.