Expansion
Transudate is pressure, exudate is permeability
Expansion
Transudates, driven by pressure and oncotic forces, usually bilateral:
- Heart failure, much the commonest
- Cirrhosis with hypoalbuminaemia
- Nephrotic syndrome
- Hypoalbuminaemia, peritoneal dialysis, hypothyroidism
- Meigs syndrome, an ovarian fibroma with ascites and effusion
- Constrictive pericarditis
Exudates, driven by inflammation and increased permeability, usually unilateral:
- Infection: parapneumonic effusion, empyema, tuberculosis
- Malignancy: lung, breast, lymphoma, mesothelioma
- Pulmonary embolism, which may be either but is usually exudative
- Connective tissue disease: rheumatoid arthritis, lupus
- Pancreatitis, with a high pleural amylase
- Post cardiac injury (Dressler’s), asbestos exposure, drugs, oesophageal rupture
“Bilateral suggests a systemic cause; unilateral suggests a local one.”
Light’s criteria separate the two, and a pH under 7.2 in an infected effusion is the threshold for chest drainage.
Every unilateral effusion warrants a diagnostic tap unless the cause is unambiguous. Cytology is positive in only about 60 per cent of malignant effusions, so a negative result does not exclude malignancy and thoracoscopy may be needed.