The Need for a Revision of Fluoroquinolone Breakpoints for Interpretation of Antimicrobial Susceptibility Testing of Feline Bacterial Isolates

The fluoroquinolone antimicrobial agents, enrofloxacin and marbofloxacin, were approved in the United States for cats in 1990 and 2001, respectively. In 2023, revised breakpoints for testing isolates from dogs were published. These canine breakpoints are discordant with the current feline breakpoints. This study was aimed at suggesting new feline breakpoints using a pharmacokinetic-pharmacodynamic (PK-PD) approach and new pharmacokinetic data. The PK-PD derived cutoff values (CO_PD) were compared to microbiologic data available for testing the susceptibility of targeted pathogens since the original approval. Compared to the current Clinical and Laboratory Standards Institute (CLSI) breakpoints for enrofloxacin and marbofloxacin in cats, these revised breakpoints are lower by two dilutions for the Enterobacterales, Pseudomonas aeruginosa, Staphylococcus spp., and Pasteurella multocida. Isolates that may have previously tested susceptible (S) may test resistant (R) using these suggested breakpoints. We also are suggesting a susceptible dose-dependent (SDD) category for testing marbofloxacin against these isolates from cats that allows for a higher dose. These suggested breakpoints may be considered by laboratories, standard-setting organizations, and industry sponsors of these antimicrobials for testing common bacteria isolated from cats.