Abstract
BACKGROUND: Influenza causes seasonal epidemics worldwide. Bacterial coinfection is a serious complication that, due to diagnostic uncertainty, often results in empirical antibiotic overuse. We aimed to identify clinical and biochemical factors associated with antibiotic initiation and bacterial coinfection in adults with confirmed influenza at the emergency department. METHODS: Adult patients presenting to the emergency department with laboratory-confirmed influenza were included across three influenza seasons (2017–2020). Clinical and biochemical data were collected through medical chart review. RESULTS: A total of 174 influenza-positive patients were included. Independent predictors of antibiotic initiation were clinical scores NEWS2 ≥ 7 (aOR 8.99, 95% CI 2.38–33.96) and CRB-65 ≥ 2 (aOR 13.22, 95% CI 2.70–64.73), as well as biomarkers CRP (aOR 1.57, 95% CI 1.21–2.02), and white blood cell count (aOR 1.25, 95% CI 1.07–1.46). Only CRP was independently associated with bacterial coinfection (aOR 1.47, 95% CI 1.16–1.86). A CRP threshold ≥ 20 mg/L demonstrated 100% sensitivity for detecting bacterial coinfection. CONCLUSION: NEWS2, CRB-65, CRP, and white blood cell count were independently associated with antibiotic initiation in adults with influenza, while only CRP predicted bacterial coinfection. Our findings could improve antibiotic stewardship strategies by underscoring the limited predictive value of clinical scores and the optimal use of biomarkers. CLINICAL TRIAL NUMBER: Not applicable. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12879-026-13166-0.