Major types of pleural effusions
Transudative pleural effusions are usually caused by increased systemic or pulmonary capillary pressure and decreased oncotic pressure, resulting in increased filtration and decreased absorption of pleural fluid. Major causes include cirrhosis, congestive heart failure, renal failure, and hypoalbuminemia. Protein levels are less than 3 g/dL.
Exudative pleural effusions occur when the pleural surface is damaged with associated capillary leak and increased permeability to protein or when there is decreased lymphatic drainage or decreased pleural pressure. Major causes include infection, noninfectious inflammatory disease, malignancy, recent surgery, and acute pulmonary embolism. Protein levels are greater than 3 g/dL, the pleural protein-to-serum protein ratio is greater than 0.5, and the pleural lactate dehydrogenase-to-serum lactate dehydrogenase ratio is greater than 0.6.