Comparative evaluation of disc diffusion and Liofilchem™ MTS strip methods with broth microdilution for cefepime/enmetazobactam susceptibility testing.

Abstract

Background: Cefepime/enmetazobactam is a newly approved β-lactam/β-lactamase inhibitor combination with promising activity against MDR Gram-negative Enterobacterales, particularly ESBL- and OXA-48-producing isolates. Reliable susceptibility testing methods are essential to guide its clinical use.

Objectives: To evaluate the performance of two commercial cefepime/enmetazobactam susceptibility testing methods, disc diffusion and Liofilchem™ MTS gradient strips, using broth microdilution (BMD) as the gold standard.

Methods: A total of 291 carbapenem-resistant Enterobacterales isolates, including 194 carbapenemase producers, were included. Susceptibility testing was performed using BMD, disc diffusion and Liofilchem™ MTS strips. Results were interpreted following EUCAST and FDA 2025 breakpoints. Essential agreement (EA) and bias were calculated for gradient strip methods according to the ISO 20776-2021 guideline, whereas categorical agreement (CA), very major errors (VMEs) and major errors (MEs) were determined for disc diffusion according to the ISO 20776-2:2007 guideline.

Results: The disc diffusion method demonstrated high CA (93.8% EUCAST, 95.9% FDA). VME rates exceeded acceptable thresholds using EUCAST breakpoints (20.8%) but were within limits for FDA (1.9%). Liofilchem™ MTS strips achieved 88.3% (95% CI: 84.1%-91.5%) EA and a bias of -8.9%. Both methods accurately detected susceptibility in OXA-48-producing isolates but showed limitations for isolates close to the MIC breakpoints.

Conclusions: Disc diffusion and Liofilchem™ MTS strips represent reliable alternatives to BMD for routine cefepime/enmetazobactam testing.