Pleural effusion is defined as excess fluid accumulation in the pleural space. It can result from increased pleural fluid formation in the lung interstitium, parietal pleura, or peritoneal cavity, or from decreased pleural fluid removal by the parietal pleural lymphatics.

The two major classes of pleural effusions are transudates, which are caused by systemic influences on pleural fluid formation or resorption, and exudates, which are caused by local influences on pleural fluid formation and resorption. Common causes of transudative effusions are left ventricular heart failure, cirrhosis, and nephrotic syndrome. Common causes of exudative effusions are pneumonia, malignancy, and pulmonary embolism. A more comprehensive list of the etiologies of transudative and exudative pleural effusions is provided in Table 142-1.

Exudates fulfill at least one of the following three criteria: high pleural fluid/serum protein ratio (>0.5), pleural fluid lactate dehydrogenase (LDH) greater than two-thirds of the laboratory normal upper limit for serum LDH, or pleural/serum LDH ratio >0.6. Transudative effusions typically do not meet any of these criteria. For exudative effusions, pleural fluid should also be tested for pH, glucose, white blood cell count with differential, microbiologic studies, cytology, and amylase. An algorithm for determining the etiology of a pleural effusion is presented in Fig. 142-1.

Despite full evaluation, no cause for the pleural effusion will be found in 25% of pts; many of these effusions are likely due to viral infections. A subset of the most common types of pleural effusions is described in the following sections.