The impact of commercially available media on cefiderocol susceptibility testing by broth microdilution method

The reference method for determining cefiderocol MIC is broth microdilution (BMD) using iron-depleted cation-adjusted Mueller-Hinton broth (ID-CAMHB). However, laboratories have reported variable MIC findings and difficulties in determining MIC endpoints. To investigate this, the BMD method was re-evaluated by examining the chelation time for iron depletion of the media and by testing reproducibility using four MHB sources (BD-BBL, BD-Difco, Oxoid, and Merck) and 85 Gram-negative isolates (<i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, <i>Pseudomonas aeruginosa</i>, and <i>Acinetobacter baumannii</i>), of which 63 were previously evaluated <i>in vivo</i>. Extending chelation time to 6 h provided optimal iron reduction to ≤0.03 µg/mL for all media. MIC reproducibility was high for each medium source and varied by species (93.3%-99.0% of MIC values within one dilution of modal MIC values); however, different sources of ID-CAMHB produced (reproducible) different modal MIC values for isolates. ID-CAMHB from BD-BBL and BD-Difco showed the best correlation with 96.5% of isolates showing modal MIC values within one dilution between media. MIC values generated with these two media were the most predictive of efficacy <i>in vivo</i>. When trailing was observed, it was seen across all media, and refined reading guidelines were applied to enhance reproducibility of determining the MIC endpoint. The 6-h chelation and the refined reading guidelines for trailing are now incorporated into the Clinical and Laboratory Standards Institute M100 document. When using the BMD method to determine cefiderocol susceptibility, the impact of media should be considered, and based on these results, ID-CAMHB prepared with BD-BBL or BD-Difco is recommended.IMPORTANCEThe reference antibiotic susceptibility testing method for cefiderocol is the broth microdilution method approved by the Clinical and Laboratory Standards Institute in 2016, using iron-depleted cation-adjusted Mueller-Hinton broth. A few manual devices have come to the market replicating this method, but inconsistent results with these devices have been reported. The current study identified the source of Mueller-Hinton broth as a variable in cefiderocol MIC determinations and provides detailed description of the preparation of iron-depleted cation-adjusted Mueller-Hinton broth, and revised reading guidance to improve reproducibility of cefiderocol MIC determinations for <i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, <i>Pseudomonas aeruginosa</i>, and <i>Acinetobacter baumannii</i>. The recommendations made in this study should enhance the reproducibility of cefiderocol broth microdilution susceptibility testing.