Pleural effusions in the atypical pneumonias.

Patients with atypical pneumonias, whether caused by bacterial, fungi, or viruses are associated with pleural effusions. The effusions generally are small and ipsilateral to the parenchymal infiltrate. Usually, the pleural fluid is a serous exudate with a predominance of mononuclear cells. The pleural fluid glucose and, presumably, the pleural fluid pH are not low. The etiologic organism has been isolated from pleural fluid but usually is not necessary to establish the diagnosis. Pleural biopsy is not helpful in diagnosis with the exception of acute coccidioidal pneumonia and effusion. Pleural effusions in the atypical pneumonias are more common than generally appreciated, rarely provide a definitive diagnosis, and resolve spontaneously with treatment of the pneumonia without pleural space manipulation. Furthermore, evidence of pleural effusion is a marker of the atypical pneumonias and directs the clinician along the appropriate diagnostic pathway based on the clinical presentation.