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Open Access Highly Accessed Research article

Etiology of community-acquired pneumonia in a population-based study: Link between etiology and patients characteristics, process-of-care, clinical evolution and outcomes

Alberto Capelastegui17*, Pedro P España1, Amaia Bilbao2, Julio Gamazo3, Federico Medel4, Juan Salgado4, Iñaki Gorostiaga4, Maria Jose Lopez de Goicoechea5, Inmaculada Gorordo1, Cristobal Esteban1, Lander Altube1, Jose M Quintana6 and on behalf of Poblational Study of Pneumonia (PSoP) Group

Author Affiliations

1 Pneumology Service Hospital Galdakao, Galdakao, Bizkaia, Spain

2 Basque Foundation for Health Innovation and Research (BIOEF) - CIBERESP, Sondika, Bizkaia, Spain

3 Emergency Service Hospital Galdakao, Galdakao, Bizkaia, Spain

4 Family Practice Comarca Interior Bizkaia, Galdakao, Bizkaia, Spain

5 Microbiology Service Hospital Galdakao, Galdakao, Bizkaia, Spain

6 Research Unit Hospital Galdakao -CIBERESP, Galdakao, Bizkaia, Spain

7 Service of Pneumology, Hospital Galdakao, 48960, Galdakao, Bizkaia, Spain

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BMC Infectious Diseases 2012, 12:134  doi:10.1186/1471-2334-12-134

Published: 12 June 2012



The etiologic profile of community-acquired pneumonia (CAP) for each age group could be similar among inpatients and outpatients. This fact brings up the link between etiology of CAP and its clinical evolution and outcome. Furthermore, the majority of pneumonia etiologic studies are based on hospitalized patients, whereas there have been no recent population-based studies encompassing both inpatients and outpatients.


To evaluate the etiology of CAP, and the relationship among the different pathogens of CAP to patients characteristics, process-of-care, clinical evolution and outcomes, a prospective population-based study was conducted in Spain from April 1, 2006, to June 30, 2007. Patients (age >18) with CAP were identified through the family physicians and the hospital area.


A total of 700 patients with etiologic evaluation were included: 276 hospitalized and 424 ambulatory patients. We were able to define the aetiology of pneumonia in 55.7% (390/700). The most frequently isolated organism was S. pneumoniae (170/390, 43.6%), followed by C. burnetti (72/390, 18.5%), M. pneumoniae (62/390, 15.9%), virus as a group (56/390, 14.4%), Chlamydia species (39/390, 106%), and L. pneumophila (17/390, 4.4%). The atypical pathogens and the S. pneumoniae are present in pneumonias of a wide spectrum of severity and age. Patients infected by conventional bacteria were elderly, had a greater hospitalization rate, and higher mortality within 30 days.


Our study provides information about the etiology of CAP in the general population. The microbiology of CAP remains stable: infections by conventional bacteria result in higher severity, and the S. pneumoniae remains the most important pathogen. However, atypical pathogens could also infect patients in a wide spectrum of severity and age.

Pneumonia; Population-based study; Etiology