Fosfomycin Etest for Enterobacteriaceae : Interobserver and interlaboratory agreement

Abstract

OBJECTIVES: 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, though the reading of 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 to AD. METHODS: Etests were performed for 57 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. RESULTS: Essential Agreement (EA) and CA between Etest and AD was 57% and 89% (κ-value 0.68), respectively, with an underestimation of Etest interpretations compared to AD of 0.26 (95%CI: 0.03-0.48) 2-fold dilutions. Between Etest observations, IO-EA and -CA was reached in 82% and 94%; IL-EA and -CA in 38% and 85%; and IOIL-EA and -CA in 40% and 85% of comparisons, respectively. Agreement of the Etest to AD and between Etests was better for E.coli than for other species. Ignoring all macrocolonies and haze from Etest interpretation, improved the agreement to AD (CA κ-value 0.80) and between Etests (CA κ-value from 0.68 to 0.81). CONCLUSIONS: 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.