Colistin Susceptibility Testing of Gram-Negative Bacilli: Better Performance of Vitek2 System than E-Test Compared to Broth Microdilution Method as the Gold Standard Test

Abstract

Of 138 isolates, meropenem, colistin and dual resistance were detected in 110 (79.7%), 31 (22.5%) and 21 (15.2%) of isolates, respectively. Using the European Committee on Antimicrobial Susceptibility Testing guidelines (susceptible, ≤2 μg/ml), Vitek2 performed better than E-test (essential agreement, 92.2% vs. 63.7%; categorical agreement, 94.1% vs. 93.1%; very major error [VME], 10% vs. 23.3%). However, Vitek2 overcalled resistance than E-test (major error, 4.2% vs. 0%). Considering Chew et al. proposed breakpoints (susceptible, ≤1 μg/ml), VMEs declined for both test (6.7% vs. 10%), but still remained unacceptable. Of eight colistin-heteroresistant isolates, two VME were categorised by Vitek2, one VME was by E-test, and two were uninterpretable. Both Vitek2 and E-test are unreliable. Further studies correlating minimum inhibitory concentrations with clinical outcome are needed to determine the accurate breakpoints for better patient management.