Performance of disc diffusion and gradient strip test on different Mueller–Hinton agar media plates and ComASP® and Bruker UMIC® Cefiderocol Microdilution Panels for cefiderocol susceptibility testing in carbapenem-resistant <i>Pseudomonas aeruginosa

Background Cefiderocol antimicrobial susceptibility testing requires standardized iron levels to produce consistent results. Here we assessed the analytical performances of two commercialized broth microdilution (BMD) methods (Bruker’s UMIC and Liofilchem’s ComASP), of disc diffusion (DD) and the gradient strip method on a collection of clinical carbapenem-resistant-Pseudomonas aeruginosa (CRPA) isolates. Methods A collection of 117 WGS-CRPA was used. Cefiderocol susceptibility was determined (i) by manual BMD (reference method) using iron-depleted Mueller–Hinton (ID-MH), (ii) with the UMIC and ComASP BMD panels, (iii) by DD and (iv) using the gradient strip test, these latter two on bioMérieux MH-agar, Liofilchem MH-agar, and in-house-produced iron-depleted (ID)-MH agar plates. Results Cefiderocol MIC50 and MIC90 were 1 and 4 mg/L, respectively. Eighteen isolates (15.4%) were classified as resistant (MIC &gt; 2 mg/L) according to the standard EUCAST-based BMD. Overall, the ComASP panel showed the highest essential agreement (EA) with the standard BMD (82.1%), with a bias of −8.5%, and categorical agreement (CA) of 94%. The UMIC panel exhibited an EA of 74.4%, with a bias of 47%, and CA of 78.6%, exhibiting a tendency to overestimate the MICs. The EAs of the gradient strip test on bioMérieux-MH, Liofilchem-MH and ID-MH agar plates were 40.2%, 45.3% and 63.2%, respectively. Considering only categorizable results, the CAs of DD were 92.7%, 94.4% and 93.9%, respectively. Conclusions The commercial ComASP panel may be used for cefiderocol testing of P. aeruginosa clinical isolates in diagnostic laboratories, ideally in combination with another method, such as the EUCAST-based DD. However, the standard manual BMD should still be performed to confirm the result when MICs are close to the breakpoint.