Issues with Cefiderocol Testing: Comparing Commercial Methods to Broth Microdilution in Iron-Depleted Medium-Analyses of the Performances, ATU, and Trailing Effect According to EUCAST Initial and Revised Interpretation Criteria.

Background: The rise of multi-drug-resistant Gram-negative bacteria necessitates the development of new antimicrobial agents. Cefiderocol shows promising activity by exploiting bacterial iron transport systems to penetrate the outer membranes of resistant pathogens.

Objectives: This study evaluates the efficacy of cefiderocol testing methods and trailing effect impact using a ComASP Cefiderocol panel, disk diffusion (DD), and MIC test strips (MTS) compared to iron-depleted broth microdilution (ID-BMD).

Methods: A total of 131 Gram-negative strains from clinical samples was tested by commercial methods and the gold standard. Results were interpreted as per 2024 and 2023 EUCAST guidelines.

Results: ID-BMD revealed high cefiderocol susceptibility among Enterobacterales and , with one isolate being resistant. exhibited higher MIC values, particularly considering trailing effects that complicated MIC readings. ComASP showed 97% categorical agreement (CA) and 66% essential agreement (EA) with ID-BMD for Enterobacterales but failed to detect the resistant . DD tests demonstrated variable CA (72% or 93%), and 38% or 34% of strains within the ATU according to EUCAST Breakpoint Tables v13.0 and 14.0, respectively, with major errors only. MTS for had 100% CA but 44% EA, and often underestimated MIC values.

Conclusions: The study emphasizes the need for standardized criteria to address trailing effects and ATU and highlights the discrepancies between testing methods. While cefiderocol resistance remains rare, accurate susceptibility testing is crucial for its effective clinical use. The findings suggest that current commercial tests have limitations, necessitating careful interpretation and potential supplementary testing to guide appropriate antibiotic therapy.