Is Streptococcus pneumoniae the leading cause of pneumonia of unknown etiology? A microbiologic study of lung aspirates in consecutive patients with community-acquired pneumonia

PURPOSE: Although a wide variety of recognized pathogens can cause communit y-acquired pneumonia, in many patients the etiology remains unknown after r outine diagnostic workup. The aim of this study was to identify the causal agent in these patients by obtaining lung aspirates with transthoracic need le aspiration. SUBJECTS AND METHODS: During a 15-month period, all consecutive patients wi th community-acquired pneumonia who were eligible for transthoracic needle aspiration were enrolled in the study. In addition to conventional microbia l methods (culture of blood and sputum, serologic studies), we per formed c ultures and genetic and antigen tests for common respiratory pathogens in l ung aspirates. RESULTS: The study group consisted of 109 patients. Conventional microbial studies identified an etiology in 54 patients (50%), including Mycoplasma p neumoniae in 19 patients, Chlamydia pneumoniae in 9 patients, and Streptoco ccus pneumoniae in 9 patients. Among the remaining 55 patients, study of th e lung aspiration provided evidence of the causal agent in 36 (65%). In 4 a dditional patients with a single microbial diagnosis by conventional method s, the lung sample provided evidence of an additional microorganism. The ne w pathogens detected by lung aspiration were S. pneumoniae in 18 patients, Haemophilus influenzae in 6 patients, Pneumocystis carinii in 4 patients, a nd C, pneumoniae in 3 patients; other organisms were identified in 4 patien ts. CONCLUSIONS: In our study, S. pneumoniae was the leading cause of community -acquired pneumonia, accounting for 25% of all cases, including about one-t hird of the cases the cause of which could not be ascertained with routine diagnostic methods. (C) 1999 by Excerpta Medica, Inc.