Development of a national EUCAST challenge panel for antimicrobial susceptibility testing

A challenge panel of bacterial strains useful for clinical laboratories to validate their European Committee on Antimicrobial Susceptibility Testing (EUCAST) antimicrobial susceptibility test (AST) system was established. A total of 117 strains, obtained from Belgian Reference Centers (n=57) and from routine clinical samples (n=60) was selected based on resistance pattern. These strains were analyzed in 7 different laboratories by 3 different automated AST systems (Vitek (n=2), Phoenix (n=2) and Microscan (n=2)) and by disk diffusion from 5 different manufacturers (Rosco (n=2), Becton-Dickinson (n=2), Biomérieux (n=1), Bio-rad (n=1) and i2a (n=1)). To select the challenge panel, selection criteria were set for categorical agreement (CA) between the different systems and the number of very major errors (VME), major errors (ME) and minor errors (MI). VMEs or MEs for at least 2 antibiotics were observed in 43% of all strains, leading to the exclusion of these strains to be selected in the panel. In only 10% of all tested strains there was a 100% CA for all antibiotics. Finally, 28 strains (14 Gram-positive and 14 Gram-negative) covering a wide spectrum of resistance mechanisms were selected. Pilot-testing of this challenge panel in 20 laboratories mainly confirmed the results of the validation study. Only 6 strains withheld for the pilot-study could not be used as challenge strain due to an overall (very) major error rate of more than 5% for a particular antibiotic (n=5) or for two antibiotics (n=1). To conclude, this challenge panel should facilitate the implementation and use of EUCAST breakpoints in laboratories.