Antimicrobial susceptibility and distribution trends among AmpC-producing Enterobacterales at a regional health authority in British Columbia: a 5-year retrospective review

Introduction: Existing literature supports using cefepime as a carbapenem-sparing agent to treat AmpC-producing Enterobacterales infections. To characterize existing susceptibility trends, we conducted a 5-year retrospective study assessing minimum inhibitory concentration (MIC) distributions for ceftriaxone, cefepime, piperacillin-tazobactam, and meropenem across Vancouver Island Health Authority (Vancouver Island, British Columbia, Canada). Methods: MIC data for AmpC isolates between November 2019 to October 2024 were retrieved (BD Epicenter™). Blood cultures, invasive specimens, urine and miscellaneous samples (e.g., respiratory, wounds) were included; we excluded surveillance specimens. MIC values were interpreted based upon CLSI M100-E34 breakpoints (Table 2A-1). Descriptive statistics were computed and compared between moderate-risk AmpC inducers (MRAC) versus low-risk AmpC inducers (LRAC). Results: 5,809 isolates were analyzed, with 3,646 (62.8%) identified as MRAC. Across all organisms, susceptibility rates for ceftriaxone, piperacillin-tazobactam, cefepime, and meropenem were 81.6%, 87.4%, 95.5%, and 98.7% respectively. Compared to LRAC, MRAC organisms demonstrated lower susceptibility to ceftriaxone (76.0% versus 91.1%) and piperacillin-tazobactam (81.5% versus 97.3%). Cefepime and meropenem demonstrated similar susceptibility rates between LRAC and MRAC. MIC90 for cefepime was 1 µg/ml for all species except E. cloacae (MIC90 4µg/ml). Conclusion: Between 2019-2024, 95.5% of our isolates demonstrated susceptibility to cefepime within our local health authority. Further research will correlate patient outcomes and establish MIC thresholds to guide routine testing and clinical use of cefepime.