BACKGROUND: There is little evidence about the relation between aetiology, illness severity and clinical course of respiratory tract infections (RTI) in primary care. Understanding these associations would aid to develop effective management strategies for these infections.

AIM: To investigate whether the clinical presentation and illness course differ between RTI in whom a viral pathogen was detected and those in whom a potential bacterial pathogen was found.

DESIGN AND SETTING: Post hoc analysis of data from a pragmatic randomised trial on the effects of oseltamivir in patients with influenza-like illness (ILI) in primary care (n=3266) in 15 European countries.

METHODS: Patient characteristics, signs and symptoms were registered at baseline. Naso-pharyngeal (adults) or nasal and pharyngeal (children) swabs were taken for PCR analysis. Patients were followed up until 28 days after inclusion. Regression models and Kaplan-Meier curves were used to analyse the relation between aetiology, clinical presentation at baseline and course of disease including complications.

RESULTS: Except for a less prominent congested nose (OR 0.55, CI 0.35 - 0.86) and acute cough (OR 0.52, CI 0.27 - 0.65) in ILI patients in whom a possible bacterial pathogen was isolated, there were no clear clinical differences in presentations between those with a possible bacterial aetiology than in those with a viral one. Also the course of disease and complications were not related to aetiology.

CONCLUSION: Given the currently available microbiological tests and antimicrobial treatments, and outside pandemics like COVID-19, microbiological testing in primary care patients with ILI seems to have limited value.