Clinical characteristics of microbiologically inappropriate empiric antibiotics in emergency department patients with bacteremia

Septic patients frequently receive empiric antibiotics before microbiological results are available, and some regimens fail to cover the eventual causative pathogen. Such microbiologically inappropriate empiric therapy has been associated with worse clinical outcomes; however, the patterns and contributing circumstances leading to these mismatches in real-world emergency care remain insufficiently described. We conducted a single-center retrospective study of adult patients with bacteremia who received empiric intravenous antibiotics in a Japanese emergency department from January to December 2024. Microbiologically inappropriate empiric therapy was defined as the absence of in vitro susceptibility of the isolated bloodstream pathogen to the initial regimen. Clinical characteristics, infection source estimation, and pathogen profiles were examined descriptively. Among 118 patients with bacteremia who met inclusion criteria, 11 (9.3%) received microbiologically inappropriate empiric therapy. The median age was 82 years, and all patients met sepsis-3 criteria. Most mismatches were attributable either to infections caused by extended-spectrum β-lactamase–producing Enterobacterales despite correct initial source estimation or to inaccurate presumptive identification of the infectious source, which led to the selection of an ineffective empiric regimen. Mortality was more frequent among patients with incorrect initial source estimation. In conclusion, 2 distinct patterns contributed to inadequate empiric antibiotic coverage: resistant pathogens and diagnostic uncertainty regarding the infectious source. These findings suggest that antimicrobial stewardship efforts in emergency settings should not focus solely on antimicrobial spectrum selection but must also incorporate strategies to improve early source identification and timely recognition of resistant organisms at the point of care.