Background: Very few studies have addressed the etiology of community-acquired pneumonia (CAP) treated in an ambulatory setting.
Methods: Patients were recruited from physicians' offices and from Emergency Rooms in Canada. Pneumonia was defined as two or more respiratory symptoms and signs and a new opacity on chest radiograph interpreted by a radiologist as pneumonia. Blood and sputum for culture as well as acute and convalescent serum samples for serology were obtained. Antibodies to Mycoplasma pneumoniae and Chlamydia pneumoniae were determined using enzyme-linked immunosorbent assays.
Results: Five hundred and seven patients were enrolled in the study; 419 (82%) had blood cultures done, seven (1.4%) of which were positive for Streptococcus pneumoniae; 241 (47.5%) had a sputum processed for culture, 31% of which were positive for a potential respiratory pathogen. 437 (86.2%) had both acute and convalescent serum samples obtained, 148 (33.8%) of which gave a positive result. Overall an etiological diagnosis was made in 48.4% of the patients. M. pneumoniae accounted for 15% of the cases, C. pneumoniae 12%, S. pneumoniae 5.9% and Haemophilus influenzae 4.9%.
Conclusions: Despite considerable effort an etiological diagnosis of CAP treated on an ambulatory basis was made in only half the patients. The most commonly identified pathogens were M. pneumoniae, C. pneumoniae, S. pneumoniae,