Impact of medium composition on cefiderocol susceptibility testing: comparison of CLSI/EUCAST-compliant and commercial broth microdilution methods.

Cefiderocol is a siderophore cephalosporin active against multidrug-resistant Gram-negative bacteria. However, accurate cefiderocol MIC determination remains challenging due to methodological variability. This study investigated whether differences between a CLSI/EUCAST-compliant in-house BMD method and the commercial UMIC system influence MIC determination and susceptibility interpretation. A total of 119 clinical isolates, including 44 Escherichia coli and 75 Klebsiella pneumoniae, collected by the German National Reference Centre for Multidrug-resistant Gram-negative Bacteria between September 2022 and September 2024, were analysed. DDT inhibition zone diameters ranged from 6 to 30 mm. Cefiderocol MICs were determined using an in-house BMD reference method and the commercial UMIC system and compared according to EUCAST breakpoints. For isolates with DDT zones <21 mm, in-house BMD classified 35 isolates as susceptible and 27 as resistant, whereas UMIC classified 31 as susceptible and 31 as resistant. Isolates with DDT zones within the EUCAST area of technical uncertainty (ATU, 21-23 mm) and ≥24 mm showed high categorical concordance across MIC methods. Overall categorical agreement and essential agreement between the in-house BMD method and the commercial UMIC system were 81.5% and 60.5%, respectively. In-house BMD MICs clustered at 1-4 mg/L, whereas UMIC MICs showed broader distributions. These findings coincided with differences in calcium, magnesium and zinc concentrations. Method-dependent variability in cefiderocol MIC determination may affect susceptibility interpretation near clinical breakpoints. Differences in testing conditions, particularly medium composition, may contribute to the observed differences. DDT showed good categorical concordance with both BMD methods for clearly susceptible isolates.