Multicentre interlaboratory analysis of routine susceptibility testing with a challenge panel of resistant strains.

In order to elaborate a new national challenge panel of resistant Gram-negative bacilli and Gram-positive cocci strains for the validation of routine antimicrobial susceptibility testing (AST) methods, an interlaboratory evaluation was organised. The results of 12 well-characterised multidrug-resistant strains tested by nine laboratories using local disk diffusion (DD) and automated AST (AUST) methods were compared with the reference broth microdilution method. Overall categorical agreement ranged from 70% to 100% both for DD and AUST and was >90% for all but one strain for all antibiotics. Our multicentre AST study showed good reproducibility and the panel can be used as national resistant reference strains for routine AST validation.