Etiology of community-acquired pneumonia: increased microbiological yield with new diagnostic methods.

Background: The microbial etiology of community-acquired pneumonia (CAP) is still not well characterized. During the past few years, polymerase chain reaction (PCR)-based methods have been developed for many pathogens causing respiratory tract infections. The aim of this study was to determine the etiology of CAP among adults-especially the occurrence of mixed infections among patients with CAP-by implementing a new diagnostic PCR platform combined with conventional methods.

Methods: Adults admitted to Karolinska University Hospital were studied prospectively during a 12-month period. Microbiological testing methods included culture from blood, sputum, and nasopharyngeal secretion samples; sputum samples analyzed by real-time quantitative PCR for Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis; nasopharyngeal specimens analyzed by use of PCR; serological testing for Mycoplasma pneumoniae, Chlamydophila pneumoniae, and viruses common in the respiratory tract; and urine antigen assays for detection of pneumococcal and Legionella pneumophila antigens.

Results: A microbial etiology could be identified for 67% of the patients (n = 124). For patients with complete sampling, a microbiological agent was identified for 89% of the cases. The most frequently detected pathogens were S. pneumoniae (70 patients [38%]) and respiratory virus (53 patients [29%]). Two or more pathogens were present in 43 (35%) of 124 cases with a determined etiology.

Conclusions: By supplementing traditional diagnostic methods with new PCR-based methods, a high microbial yield was achieved. This was especially evident for patients with complete sampling. Mixed infections were frequent (most commonly S. pneumoniae together with a respiratory virus).