Comparison of Four Methods for Colistin Susceptibility Testing Against Multidrug Resistant Gram-Negative Bacteria in a Bulgarian University Hospital

The aim of this study was to compare the performances of four colistin susceptibility testing methods against 200 multidrug-resistant Gram-negative bacteria. The study was conducted against clinical isolates of Klebsiella pneumoniae (97 isolates), Escherichia coli (41), Enterobacter cloacae complex (13), Pseu¬domonas aeruginosa (21), and Acinetobacter baumannii complex (28). Results, obtained from the broth microdilution method (BMD), performed according to ISO standard 20776-1, the commercially available broth microdilution kit - MIKROLATEST MIC Colistin (MLTCol, ErbaLachema), colistin pre-diffusion method kit (PDCol, ROSCO Diagnostica A/S), the rapid Polymixin NP test (RPNP) and the Superpolymyx¬in medium (SPM) were compared to BMD as a referent method. According to the BMD method, 16.5% of tested isolates were colistin-resistant. The essential agreement (EA) between MLTCol and the BMD was 90.5% (134/148), categorical agreement (CA) was 98.5% (197/200), major discrepancies (MD) were 3 (1.8%), and there were no very major discrepancies (VMD). The CA between RPNP and BMD was 98.7% (149/151), with no MD and VMD at 6.7%. Compared with the BMD, the specificity, and sensitivity of RPNP were 100% and 93.3%, respectively. The PDCol had CA of 95.5 % (191/200) with the BMD, with VMD at 3.0%, and MD at 4.8%. CA between the SPM and BMD was observed in 94% (188/200), and there were no VMD and MD in 7.2%, nonetheless, the SPM detected 12 heteroresistant isolates of K. pneumoni¬ae, categorized as susceptible by the referent BMD. In conclusion, the agar-based methods better detected some heteroresistant isolates and could be used in addition to BMD to improve their recognition.