Abstract Purpose Bloodstream infections (BSI) are associated with high mortality. Previous studies have reported worse outcome for polymicrobial than for monomicrobial BSIs. We analyzed patient characteristics and temporal trends of the incidence and outcome of polymicrobial BSIs in Finland during 2004–2018. Methods We used data from national registries to identify polymicrobial BSIs during 2004–2018 and to determine origin of infection, patients’ comorbidities and death within 30 days. Charlson comorbidity index (CCI) was calculated according to ICD-10 diagnose codes. Results In total, 173,715 BSIs were identified; 11,347 (6.5%) were polymicrobial. Compared with monomicrobial BSIs, the proportion of males, healthcare-associated BSIs, and patients with high CCI were greater in polymicrobial BSIs (58.5% vs. 51.5%, 34.7% vs. 28.7%, and 24.9% vs. 21.1%, respectively). Escherichia coli , enterococci, coagulase-negative staphylococci, and Klebsiella sp. were the most common pathogens of polymicrobial BSIs. Anaerobic bacteria were noted in 16.3% of polymicrobial BSIs, compared with 4.3% of monomicrobial BSIs. The annual polymicrobial BSI incidence rose from 9.7 to 21.8/100,000 population during 2004–2018, most sharply among patients aged ≥ 90 years. The 30-day case fatality of polymicrobial BSIs was 20.6%, significantly higher than in monomicrobial BSIs (12.4%), and a decline from 25.2 to 20.8% was observed over time. Conclusion Polymicrobial BSI incidence increased twofold during 2004–2018. The case fatality was considerably higher in polymicrobial than in monomicrobial episodes, likely related to patients’ older age and more severe comorbidity. Our findings emphasize the need for prompt recognition of patients at risk to guide the choice of empiric treatment.
Kontula et al. (2025) studied this question.