Impact of inter-lab variation on the estimation of epidemiological cut-off values for disc diffusion susceptibility test data for Aeromonas salmonicida

Two laboratories investigated the susceptibility of 106 Aeromonas salmonicida strains (from Denmark, France, Ireland, Norway and Scotland) to erythromycin, gentamicin, oxytetracycline and oxolinic acid using the disc diffusion protocols (M42-A) published by the Clinical and Laboratory Standards Institute. In studies of susceptibility to florfenicol an additional 15 Canadian strains were included. Comparison of the data generated by the two laboratories demonstrated that for each disc both detected a similar pattern of distribution but that there was a significant numerical difference in the zone sizes they recorded. Analysis of the extent of this lateral shift between the data generated in two laboratories indicated that the application of a single laboratory-independent epidemiological cut-off value for each disc could result in disagreement between the laboratories as to whether a strain should be classified as wild-type or non wild-type. Normalised resistance interpretation was employed to generate epidemiological cut-off values from the data obtained by each laboratory. The use of these laboratory-specific cut-off values resulted in both laboratories achieving complete agreement as to the classification of all strains to all agents.