Veterinary-specific breakpoints for ceftiofur applicable to canine <i>Escherichia coli</i>

ABSTRACT Ceftiofur is a third-generation cephalosporin approved for veterinary applications and is used to treat Escherichia coli infections in dogs, particularly urinary tract infections (UTIs), in countries including China. Despite this use, there are no approved breakpoints for interpretation of ceftiofur susceptibility testing results. This absence limits the guidance that veterinarians need to effectively prescribe ceftiofur in dogs. It also limits the analysis of data from resistance monitoring and surveillance programs. This study aimed to establish ceftiofur interpretive categories and breakpoints that can be used to assess antimicrobial susceptibility testing results for canine E. coli isolates. Our methods included a definition of the wild-type cutoff (CO WT ) based on resistance phenotypes from two sources, deriving the pharmacokinetics/pharmacodynamics (PK-PD) cutoff (CO PD ) through PK-PD analyses and Monte Carlo simulations and determining the clinical cutoff (CO CL ) from a trial in dogs. Minimal inhibitory concentration (MIC) data were collected from China and the European database (European Committee on Antimicrobial Susceptibility Testing [EUCAST]). CO WT was determined as 1 μg/mL. The CO PD was 0.25 μg/mL. Through a canine cystitis trial, a CO CL of 2 μg/mL was determined. Based on these findings and Clinical and Laboratory Standards Institute (CLSI)-recommended methods, we propose urinary tract infection-specific MIC breakpoints of ≤1 µg/mL (susceptible), 2 μg/mL (intermediate), and ≥4 µg/mL (resistant) and corresponding disk diffusion zone diameter breakpoints, determined by the error rate-bounded (ERB) method, of ≥23 mm (susceptible), 20–22 mm (intermediate), and ≤19 mm (resistant). The systemic breakpoints (non-urine) were determined to be ≤0.125 µg/mL (susceptible), 0.25 μg/mL (intermediate), and ≥0.5 µg/mL (resistant). IMPORTANCE Ceftiofur can be legally used, but laboratories and veterinarians lack canine-specific breakpoints to determine whether an isolate is susceptible or resistant. Without such criteria, test results can be inconsistent, treatment choices uncertain, and early signs of emerging resistance overlooked. This study fills that gap by defining evidence-based, canine-specific breakpoints for ceftiofur, particularly for urinary tract infections (UTIs). Adoption of these breakpoints will allow more consistent reporting by diagnostic laboratories, improve therapeutic decision-making for veterinarians, and provide a reliable basis for resistance surveillance. These advances strengthen the One-Health antimicrobial stewardship and help preserve the effectiveness of important antimicrobials for companion animals and the wider community.