Challenges to accurate susceptibility testing and interpretation of quinolone resistance in Enterobacteriaceae: results of a Spanish multicentre study

Results: Susceptibility testing in the participating centres was frequently performed using the MicroScan WalkAway, Vitek 2 and Wider systems (48%, 30% and 8%, respectively). CLSI/EUCAST breakpoints were used in 71%/29% of the determinations. The percentage of VMEs for all quinolones was well below 2%. Only ofloxacin and moxifloxacin showed higher values for raw VMEs (6.6%), which decreased to 0% and 2.9%, respectively, in the interpreted VMEs. These errors were particularly associated with the CC-03 strain [qnrS2 +aac(6 ′ )-Ib-cr]. For MEs, percentages were always ,10%, except in the case of ofloxacin and nalidixic acid. There was a significantly higher percentage of all types of errors for strains whose MICs were at the border of clinical breakpoints. Conclusions: The use of different breakpoints and methods, the complexity of mutation-driven and transferable resistance mechanisms and the absence of specific tests for detecting low-level resistance lead to highvariability and represent a challenge to accuracy in susceptibility testing, particularly in strains with MICs on the border of clinical breakpoints.