The Achilles' heel of the Trojan Horse? A systematic evaluation of cefiderocol susceptibility testing

Cefiderocol, an innovative siderophore cephalosporin, presents a novel therapeutic option against a spectrum of multidrug-resistant (MDR) Gram-negative pathogens. Nevertheless, resistance remains a formidable challenge, particularly among metallo-beta-lactamase (MBL)-producing organisms. Accurate antimicrobial susceptibility testing (AST) for cefiderocol is complex due to the labour-intensive broth microdilution (BMD) reference method requiring iron-depleted media, lacking reproducibility. In response, commercial AST methods, including BMD panels, disc diffusion (DD), and gradient diffusion test, have been developed. Commercial BMD panels, such as ComASP® and UMIC®, demonstrate potential, with the latter reaching categorical agreement (CA) above 90%. Yet, essential agreement (EA) remains between 75% and 85%, below the 90% desired threshold, with very major errors (VMEs) occurring frequently (∼15%). Disc diffusion (DD) methods, while practical, often overcall resistance, leading to major errors (MEs) with a median across studies of 29%. Among disc manufacturers, MASTDISCS® performed best, with a pooled CA of 93.2%, 5.4% ME and 6.3% VME. Overall, discs recorded a CA of 79.4%, MEs of 29.0% and VMEs of 13.9%. Gradient diffusion tests performed least favourably among all methods, exhibiting a notably high VME rate of 41.1%, and their use should be limited. Cefiderocol AST is further complicated by disparities between European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Clinical and Laboratory Standards Institute (CLSI)/Food and Drug Administration (FDA) breakpoints, contributing to inconsistencies in susceptibility categorization across laboratories. Additional challenges, such as trailing endpoints and microcolonies within inhibition zones, further confound readings, especially in DD assays. Consequently, the standardization and rigorous validation of the best performing cefiderocol AST methodologies are imperative to ensure reliable susceptibility outcomes and optimized outcomes for patients with MDR infections.