Performance of disc diffusion and E-test on different Muller-Hinton agar media plates and ComASPⓇ and Bruker UMICⓇ Cefiderocol Microdilution Panels for Cefiderocol susceptibility testing in carbapenem-resistant Pseudomonas aeruginosa

BACKGROUND: Cefiderocol is effective against carbapenem-resistant Pseudomonas aeruginosa (CRPA) and requires standardized iron concentrations for accurate susceptibility testing. This study evaluated the performance of two commercial broth microdilution (BMD) methods (Bruker's UMIC and Liofilchem's ComASP) along with disc diffusion and gradient strip tests on a range of diverse clinical CRPA isolates. METHODS: We analyzed 117 CRPA isolates (58 carbapenemase producers, 59 non-producers) using manual BMD with iron-depleted Mueller-Hinton broth (ID-MH), UMIC and ComASP BMD panels, and disc diffusion and E-tests on bioMérieux-MH-agar, Liofilchem-MH-agar, and ID-MH-agar plates. RESULTS: Cefiderocol MIC50 and MIC90 values were 1 mg/L and 8 mg/L, respectively. Twenty-two isolates were resistant (MIC >2 mg/L, see Figure 1). As shown on Figure 2, none of the MIC-based methods achieved the established criteria for acceptable accuracy (essential agreement, EA ≥90%, and bias ±30%). The ComASP panel showed the highest EA (68.1%) with a -6% bias and 85.3% categorical agreement (CA). The UMIC panel had an EA of 55.6%, a bias of 34.2%, and a CA of 74.4%, tending to overestimate MICs. Gradient strip test CAs were 81.0% on bioMérieux-MH, 78.6% on Liofilchem-MH, and 77.8% on ID-MH agar. Disc diffusion CAs were 81.8%, 83.2%, and 83.8%, respectively. Importantly, both gradient strip and disc diffusion failed to detect most resistant isolates. CONCLUSIONS: The ComASP panel is suitable for CRPA cefiderocol susceptibility testing, ideally in combination with another method like EUCAST-based disc diffusion. However, confirming results with the standard BMD method is highly recommended when MICs are near the breakpoint.