> Broth Microdilution Testing
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Given cefiderocol’s unique mechanism of bacterial cell entry, susceptibility testing with broth microdilution requires iron-depleted cation-adjusted Mueller Hinton Broth (ID-CAMHB) according to the CLSI M100 document.1 Resources are also available for test verification.
How to Test
Devices for Cefiderocol Broth Microdilution Testing
FDA-cleared manual cefiderocol antimicrobial susceptibility testing devices available for purchase:
| Broth microdilution product | Catalog/reference number |
|---|---|
| Liofilchem™️ ComASP® Cefiderocol Microbroth Dilution Panel Test Kits | 75009U |
|
Thermo Scientific™️ Sensititre™️ MIC Plates NOTE: This item was unavailable as of August 2025. Please contact a local Thermo Fisher Scientific™ Microbiology representative for current availability. |
MDRGN2F |
ComASP Reading Guide
How to interpret the ComASP® Cefiderocol 0.008-128 quantitative broth microdilution panel from Liofilchem:
Reproduced with permission from Liofilchem. No endorsement implied.
Preparation of Iron-Depleted Cation-Adjusted Mueller Hinton Broth for In-House Cefiderocol Susceptibility Testing
Infographic and animated video describing how to prepare ID-CAMHB for cefiderocol susceptibility testing according to the CLSI M100 document:
Video by DeJonge et al. © 2025 DeJonge et al. Licensed under CC BY 4.0. No endorsement implied. Available at https://journals.asm.org/doi/10.1128/jcm.00471-25.
Determination of Cefiderocol MIC Values Using the Broth Microdilution Method
Animated video showing how to assign cefiderocol MIC values when using broth microdilution, as well as how to interpret trailing:
Video by DeJonge et al. © 2025 DeJonge et al. Licensed under CC BY 4.0. No endorsement implied. Available at https://journals.asm.org/doi/10.1128/jcm.00471-25.
MIC QC Ranges for Cefiderocol
Benchmarks to support reliable susceptibility testing and ensure compliance with CLSI performance standards1:
| MIC QC range (μg/mL)1,a | |
|---|---|
| E. coli ATCC 25922 | 0.06–0.05 |
| P. aeruginosa ATCC 27853 | 0.06–0.05 |
aQC ranges reflect MICs obtained when CAMHB is iron depleted. Chelation is used for iron depletion, which also removes other cations (ie, calcium, magnesium, and zinc). Following this process, cations are added back to concentrations of calcium 20–25 mg/L, magnesium 10–12.5 mg/L, and zinc 0.5–1.0 mg/L.
MIC-minimum inhibitory concentration; QC=quality control.
How to Interpret Results
FDA and CLSI Breakpoints for Cefiderocol
| FDA identified breakpoints2 | Minimum inhibitory concentrations (µg/mL) | ||
|---|---|---|---|
| Susceptible | Intermediate | Resistant | |
| Enterobacteralesa,b | ≤4 | 8c | ≥16 |
| Pseudomonas aeruginosa | ≤1 | 2 | ≥4 |
| Acinetobacter baumanii complex | ≤1 | 2 | ≥4 |
| Stenotrophomonas maltophilia | ≤1e | – | – |
| CLSI identified breakpoints1,d | Minimum inhibitory concentrations (µg/mL) | ||
|---|---|---|---|
| Susceptible | Intermediate | Resistant | |
| Enterobacterales | ≤4 | 8c | ≥16 |
| Pseudomonas aeruginosa | ≤4 | 8c | ≥16 |
| Acinetobacter baumanii complex | ≤4 | 8 | ≥16 |
| Stenotrophomonas maltophiliae | ≤1 | – | – |
aClinical efficacy was shown for Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, and Enterobacter cloacae complex in patients with cUTI.
bClinical efficacy was shown for Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae complex, and Serratia marcescens in patients with HABP/VABP.
cDesignation for agents that have the potential to concentrate in the urine.
dThe accuracy and reproducibility of cefiderocol testing results by disk diffusion and broth microdilution are markedly affected by iron concentration and inoculum preparation, and may vary by disk and media manufacturer. Depending on the type of variance observed, false-resistant or false-susceptible results may occur. Testing subsequent isolates is encouraged. Discussion with prescribers and antimicrobial stewardship members regarding the potential for inaccuracies is recommended.
eBreakpoints are based on PK/PD properties, MIC distributions, and limited clinical data.
CLSI=Clinical and Laboratory Standards Institute; cUTI=complicated urinary tract infection; FDA=US Food and Drug Administration; HABP/VABP=hospital-acquired bacterial pneumonia/ventilator-associated bacterial pneumonia; MIC=minimum inhibitory concentration; PK/PD=pharmacokinetic/pharmacodynamic.
The Impact of Commercially Available Media on Cefiderocol Susceptibility Testing by Broth Microdilution Method
DeJonge BLM, Slover C, Nguyen ST, et al. J Clin Microbiol. 2025:e0047125.
Key Points
- Re-evaluation of the broth microdilution method establishes the need for a longer (6-hour) chelation time when preparing ID-CAMHB for cefiderocol to achieve optimal iron reduction to ≤0.03 μg/mL
- In addition, comparison of different media support a preference for BD-BBL or Difco Mueller Hinton Broth to enhance reproducibility when testing
A Multicenter Performance Evaluation of Cefiderocol MIC Results: ComASP in Comparison to CLSI Broth Microdilution
Koeth LM, DiFranco-Fisher JM, Palavecino E, et al. J Clin Microbiol. 2025:63(2):e0092624.
Key Points
- The ComASP Cefiderocol (Liofilchem, Italy) device can be used for quantitative in vitro determination of antimicrobial susceptibility using BMD
- This multicenter study compared ComASP Cefiderocol to CLSI-modified reference BMD for submission to the FDA for device clearance
- Of 2 different reading methods evaluated, the ComASP Cefiderocol reading method (100% inhibition of growth as the MIC endpoint) demonstrated robust reliability for determining cefiderocol MICs against relevant clinical isolates (essential agreement ranged from 93.7% to 96.7% for Enterobacterales,
P. aeruginosa, and A. baumannii isolates)
Abbreviations: CAMHB=cation-adjusted Mueller Hinton Broth; CLSI=Clinical and Laboratory Standards Institute; cUTI=complicated urinary tract infection; FDA=US Food and Drug Administration; HABP/VABP=hospital-acquired bacterial pneumonia/ventilator-associated bacterial pneumonia; ID-CAMHB=iron-depleted cation-adjusted Mueller Hinton Broth; MIC=minimum inhibitory concentration; PK/PD=pharmacokinetic/pharmacodynamic; QC=quality control.
References:
- CLSI. Performance Standards for Antimicrobial Susceptibility Testing, 35th Edition. M100. 2025. Accessed February 3, 2025.
- US FDA. Antibacterial Susceptibility Test Interpretive Criteria. 2024. https://www.fda.gov/drugs/development-resources/cefiderocol-injection. Accessed June 15, 2025.