Fosfomycin Etest for Enterobacteriaceae: Interobserver and interlaboratory agreement

•Most fosfomycin Etests show growth of macrocolonies in the inhibition zone.•Etest observations significantly underestimate the MIC of fosfomycin compared with the reference standard agar dilution.•Fosfomycin Etest has low interobserver-interlaboratory agreement.•Higher agreement for E. coli compared with other Enterobacteriaceae.•Ignoring all growth in the inhibition zone might improve Etest performance. The increasing use of fosfomycin requires reliable susceptibility testing in clinical practice. The reference standard, agar dilution (AD), is rarely used in routine settings. The fosfomycin Etest (BioMérieux) is frequently used, although reading MICs can be hampered by the interpretation of the growth of macrocolonies in the inhibition zone. We investigated the interobserver (IO), interlaboratory (IL), and interobserver-interlaboratory (IOIL) agreement of the fosfomycin Etest and evaluated the agreement with AD. Etests were performed for 57 extended spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae of four bacterial species (Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca and Enterobacter cloacae) in two laboratories. Photographs of fosfomycin Etests were interpreted by four observers following manufacturer's instructions. Essential agreement (EA) and categorical agreement (CA) between Etest and AD were 57% and 89% (κ-value 0.68), respectively, with an underestimation of Etest interpretations compared with AD of 0.26 (95% confidence interval [CI] 0.03-0.48) 2-fold dilutions. Between Etest observations, IO-EA and -CA were reached in 82% and 94% of comparisons; IL-EA and -CA in 38% and 85% of comparisons; and IOIL-EA and -CA in 40% and 85% of comparisons, respectively. Agreement of the Etest with AD and between Etests was better for E. coli than for other species. Ignoring all macrocolonies and haze during Etest interpretation improved the agreement with AD (CA κ-value 0.80) and between Etests (CA κ-value from 0.68 to 0.81). In this study on 57 ESBL-producing Enterobacteriaceae, IOIL agreement was low with an EA of 40% and a CA of 85%, affected most by IL agreement and to a lesser extent by IO agreement. [Display omitted]