# Characteristics, incidence and outcome of polymicrobial bloodstream infections: a nationwide population-based study, Finland, 2004–2018

## TL;DR

This study shows that polymicrobial bloodstream infections are more common and deadly than single-bacteria infections, especially in older and sicker patients.

## Contribution

The study provides nationwide population-based evidence on the rising incidence and worse outcomes of polymicrobial bloodstream infections in Finland.

## Key findings

- Polymicrobial bloodstream infections increased twofold from 2004 to 2018, with the sharpest rise in patients aged ≥90 years.
- The 30-day case fatality rate was 20.6% for polymicrobial infections, significantly higher than 12.4% for monomicrobial infections.
- Polymicrobial infections were more likely to involve healthcare-associated cases, older males, and patients with higher comorbidity indices.

## Abstract

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.

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.

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.

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.

The online version contains supplementary material available at 10.1007/s15010-025-02642-5.