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Cefiderocol susceptibility testing by disk diffusion should be performed on standard agar (ie, non-iron-depleted agar).1 Resources are also available for test verification.
How to Test
Supplies for Cefiderocol Disk Diffusion Testing
FDA-cleared manual cefiderocol antimicrobial susceptibility testing devices available for purchase:
| Product | Catalog/reference number |
|---|---|
| HardyDisk™ AST Cefiderocol |
Z9431(1 cartridge) Z9435 (5 cartridges) |
| Thermo Scientific™ Oxoid™ Discs | CT1964B |
| BD BBL™ Sensi-Disc™ Cefiderocol Discs | 232255 |
Disk Diffusion QC Ranges for Cefiderocol
Benchmarks to support reliable susceptibility testing and ensure compliance with CLSI performance standards2:
| Disk diffusion QC range (mm)2,a | |
|---|---|
| E. coli ATCC 25922 | 25–31 |
| P. aeruginosa ATCC 27853 | 22–31 |
aDisk content = 30 μg.
How to Interpret Results
FDA and CLSI Breakpoints for Cefiderocol
| FDA identified breakpoints3 | Disk diffusion (zone diameter in mm) | ||
|---|---|---|---|
| Susceptible | Intermediate | Resistant | |
| Enterobacteralesa,b | ≥16 | 9-15c | ≤8 |
| Pseudomonas aeruginosa | ≥22 | 13-21 | ≤12 |
| Acinetobacter baumanii complex | ≥19 | 12-18 | ≤11 |
| Stenotrophomonas maltophilia | ≥17 | – | – |
| CLSI identified breakpoints2,d | Disk diffusion (zone diameter in mm) | ||
|---|---|---|---|
| Susceptible | Intermediate | Resistant | |
| Enterobacteralesa,b | ≥16 | 9-15c | ≤8 |
| Pseudomonas aeruginosa | ≥18 | 13-17c | ≤12 |
| Acinetobacter baumanii complex | ≥15e | – | – |
| Stenotrophomonas maltophiliaf | ≥15 | – | – |
Disk content is 30 µg of cefiderocol.
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.
eDisk diffusion zone diameters ≤14 mm should not be interpreted or reported because zone diameters ≤14 mm occur with resistant, intermediate, and susceptible isolates. For isolates with zone diameters ≤14 mm, do not report cefiderocol without performing an MIC test.
fBreakpoints are based on PK/PD properties, MIC distributions, and limited clinical data.
CLSI=Clinical and Laboratory Standards Institute; FDA=US Food and Drug Administration.
Impact of Media Brand on Cefiderocol Disk Diffusion Results
DeMarco MG, Field AM, Donohue LE, et al. J Clin Microbiol. 2025;63(5):e0164824.
Key Points
- Use of commercial Mueller-Hinton agar for disk diffusion can result in discrepancies in antimicrobial susceptibility testing interpretation, particularly for isolates not susceptible to cefiderocol by broth microdilution
- In this analysis of multiple Mueller-Hinton agar brands, BBL (Becton Dickinson, East Rutherford, NJ) was the only agar brand that demonstrated ≥90% categorical agreement when comparing CLSI clinical breakpoint interpretations for mean zones of inhibition determined via disk diffusion to modal MICs determined via broth microdilution as the reference method
Abbreviations: 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; QC=quality control.
References:
- DeJonge BLM, et al. ASM Microbe 2025. June 19–23, 2025. Los Angeles, CA. Poster 2011.
- 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.