The usefulness of sputum culture in guiding microbiological diagnosis of community-acquired pneumonia is controversial. We evaluate and assess it using the Patients Outcome Research Team (PORT) predictive scoring system.
A cohort of 1669 patients with community-acquired pneumonia was studied. Before administering antibiotic therapy, sputum was collected and its quality evaluated. Samples were gram stained and those of good quality were assessed for a predominant morphotype (PM). Sputum cultures were processed according to standard protocols.
A sputum sample was obtained from 983 (59%) of the 1669 patients and 532 (54%) of the samples were of good quality. There was a PM in 240 (45%) of the latter samples (ie, for 14.4% of the 1669 patients) and there was no PM in 292 (55%). Culture yielded a microorganism in 207 (86%) of the 240 samples with PM and 57 (19.5%) of the 292 samples without PM (P<.05). Rates of sputum obtained, good-quality sputum specimens, PM identification, and positive culture were not significantly different among the PORT-score groups of patients (P>.05). The sensitivity and specificity of the gram-positive diplococci identification in the sputum culture of Streptococcus pneumoniae were 60% and 97.6%, and the positive and negative predictive values were 91% and 85.3%, respectively.
Good-quality sputum with PM could be obtained in only 14.4% of all patients. A PORT-score group in which sputum could be of greater usefulness in identifying the causative organism could not be identified. The presence of gram-positive diplococci in gram-stained sputum culture was highly specific for S pneumoniae.