Transudates and Exudates

Transudates are associated with increased systemic or pulmonary capillary hydrostatic pressure or decreased colloid osmotic pressure. These factors often co-exist. The pleural membranes are intact and the permeability of pleural capillaries to proteins is normal.

• Cardiac failure
• Liver failure/cirrhosis
• Renal failure, nephrotic syndrome
• Constrictive pericarditis
• Hypothyroidism
• Meig’s Syndrome (right pleural effusion with ovarian fibroma)

Exudates are associated with altered permeability of pleural membranes, increased capillary wall permeability to proteins or vascular disruption. They can also be associated with reduced or obstructed lymphatic drainage from the pleural space.

• Pneumonia
• Pulmonary embolism/infarction
• Malignancy
• Mesothelioma
• Connective tissue disorders such as rheumatoid arthritis, systemic lupus erythematosus

Aortic dissection, oesophageal rupture, pancreatitis may also cause pleural effusions. An effusion in the context of trauma should be assumed to be a haemothorax until proved otherwise.