Why Your Antibiotic Susceptibility Test Looks Different: Breakpoint Revisions and Their Clinical Impact

Presentation Description / Summary: Antibiotic susceptibility testing protocols are regularly updated as new data are available, and many clinicians are now encountering results that look different than in years past. Recent revisions to breakpoints for companion animals, most notably for fluoroquinolones and chloramphenicol, reflect updated pharmacokinetic data on drug delivery to infection sites. These changes mean that many isolates once reported as susceptible will now be categorized as resistant, often shifting treatment decisions toward higher doses or alternative therapies. For fluoroquinolones, the introduction of the susceptible-dose dependent (SDD) category requires clinicians to pair MIC results with specific dosing strategies to achieve efficacy. Chloramphenicol, long valued as a fallback for multidrug-resistant infections, has also been re-evaluated, with new data suggesting that tissue concentrations may be less effective than previously assumed. However, both fluoroquinolones and chloramphenicol achieve high urinary concentrations, and may be effective for many pathogens that would be considered resistant at other infection sites. This presentation will explain the rationale behind recent breakpoint changes, highlight their practical implications for internal medicine clinicians, and provide strategies for interpreting susceptibility reports in the context of both tissue and urinary infections. Understanding these updates is essential for making informed prescribing decisions that optimize patient outcomes while supporting antimicrobial stewardship.