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21st February 2025
Causes of pleural effusion
Effusions are traditionally separated into transudates and exudates.
The differential of effusions is wide, but the common underlying pathologies are outlined in the table below, with the most common causes in bold.
| Transudate | Exudate | Other |
|---|---|---|
| Heart failure | Infection (parapneumonic, TB) | Haemothorax |
| Cirrhosis | Malignancy (e.g. lung cancer, mesothelioma, metastatic cancer, primary effusion lymphoma) | Chylothorax (accumulation of chyle due to disruption of lymphatic drainage) |
| Hypoalbuminaemia | Autoimmune pleuritis, e.g. rheumatoid arthritis | |
| Peritoneal dialysis | PE | |
| Hypothyroidism | Benign asbestos pleural effusion | |
| Nephrotic syndrome | Post-myocardial infarction (Dressler syndrome) or post-CABG | |
| Meigs’ syndrome |
Table 12 Causes of pleural effusion
Light’s criteria can be used to differentiate between transudates and exudates. Pleural fluid is thought to be an exudate if any of the following are true:
- Pleural fluid to serum protein ratio is >0.5
- Pleural fluid to serum LDH ratio is >0.6
- Pleural fluid LDH is >⅔ of the upper limit of normal for serum LDH
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