Comparison of three standardized disc susceptibility testing methods for colistin

Objectives: With increasing antibiotic resistance in Gram-negative bacteria, the use of the polymyxins has increased in recent years. Antibiotic disc susceptibility testing remains the most widely used method in clinical laboratories, but there is very little data on the accuracy of disc testing methods for colistin. In this study, the accuracy of three standardized methods of disc susceptibility testing for colistin was compared with agar dilution. Methods: A total of 228 clinical isolates of Acinetobacter spp., Pseudomonas aeruginosa and Enterobacteriaceae were included in the study. Isolates were tested by agar dilution for susceptibility to colistin, and results were compared with those obtained by three disc susceptibility testing methods (product insert based on CLSI methodology, British BSAC and French SFM). Results: Colistin displayed good activity against Acinetobacter spp., Klebsiella spp. and Escherichia coli (MIC90 2 mg/L) but was less active against P. aeruginosa (MIC90 4 mg/L) and Enterobacter spp. (MIC90 >= 128 mg/L). Totally, 81%, 79% and 89% of colistin-resistant isolates were falsely reported as susceptible when tested by the product insert, BSAC and SFM testing methods, respectively. There were no false-resistant results. Conclusions: Disc susceptibility testing methods are unreliable at detecting colistin resistance. Dilution methods should be the method of choice for susceptibility testing of colistin.