Fosfomycin Etest for Enterobacteriaceae: Interobserver and interlaboratory agreement.

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Fosfomycin susceptibility testing is crucial due to its increasing clinical use.
• The reference agar dilution (AD) method is not routinely used.
• The fosfomycin Etest is common but interpretation is challenging due to macrocolony growth.

Purpose of the Study:

• To investigate interobserver (IO), interlaboratory (IL), and combined (IOIL) agreement of the fosfomycin Etest.
• To evaluate the agreement of the fosfomycin Etest with the reference agar dilution method.
• To assess the impact of macrocolony and haze interpretation on Etest agreement.

Main Methods:

• Etests were performed on 57 extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae across two laboratories.
• Four observers interpreted Etest results from photographs following manufacturer guidelines.
• Agreement with agar dilution and between observers/laboratories was assessed using essential agreement (EA) and categorical agreement (CA).

Main Results:

• Essential agreement (EA) and categorical agreement (CA) between Etest and AD were 57% and 89% (κ=0.68).
• Interobserver agreement was higher (EA 82%, CA 94%) than interlaboratory agreement (EA 38%, CA 85%).
• Ignoring macrocolonies and haze significantly improved Etest agreement with AD (CA κ=0.80) and between observers (CA κ=0.81).

Conclusions:

• Interobserver-interlaboratory agreement for fosfomycin Etest was low (EA 40%, CA 85%), primarily due to poor interlaboratory agreement.
• Agreement was better for *Escherichia coli* compared to other species.
• Standardized interpretation, specifically ignoring macrocolonies and haze, is recommended to improve fosfomycin Etest reliability.