Pleural Effusion
Summary
Pleural effusion is accumulation of fluid in the pleural space. Classification into transudate (protein less than 25 g/L) or exudate (protein greater than 35 g/L) using Light's criteria guides differential diagnosis. Common causes of transudates include heart failure, liver cirrhosis, and nephrotic syndrome. Exudates are caused by infection (parapneumonic, empyema, TB), malignancy, and pulmonary embolism. Diagnosis involves CXR, ultrasound, and diagnostic pleural aspiration with fluid analysis. Treatment is directed at the underlying cause. Drainage is indicated for symptomatic relief, empyema, or diagnosis.
Key Facts
- Definition: Fluid accumulation in the pleural space
- Classification: Transudate vs Exudate (Light's criteria)
- Common causes: Heart failure (transudate), malignancy, infection (exudate)
- Gold Standard Investigation: Pleural fluid analysis (protein, LDH, pH, cytology, MC&S)
- First-line Treatment: Treat underlying cause; drain if symptomatic or empyema
Clinical Pearls
Light's Criteria Pearl: Exudate if ANY of: Protein ratio greater than 0.5, LDH ratio greater than 0.6, Pleural LDH greater than 2/3 ULN.
Ultrasound Pearl: Ultrasound should be used to guide all pleural procedures - reduces complications.
pH Pearl: Pleural pH less than 7.2 in parapneumonic effusion = needs chest drain (complicated effusion/empyema).
Light's Criteria (Exudate if ANY of)
| Criterion | Exudate |
|---|---|
| Pleural fluid protein / serum protein | Greater than 0.5 |
| Pleural fluid LDH / serum LDH | Greater than 0.6 |
| Pleural fluid LDH | Greater than 2/3 upper limit of normal for serum |
Causes
| Transudate | Exudate |
|---|---|
| Heart failure | Parapneumonic/empyema |
| Liver cirrhosis | Malignancy |
| Nephrotic syndrome | TB |
| Hypoalbuminaemia | Pulmonary embolism |
| Peritoneal dialysis | Rheumatoid arthritis, SLE |
Symptoms
Signs
Imaging
- CXR: Blunting of costophrenic angle (greater than 200mL), meniscus sign
- Ultrasound: Confirms effusion, guides aspiration, characterises (simple vs complex)
- CT: Underlying cause, malignancy, loculations
Pleural Fluid Analysis
| Test | Purpose |
|---|---|
| Protein | Transudate vs exudate |
| LDH | Transudate vs exudate |
| pH | Less than 7.2 = drain (parapneumonic) |
| Glucose | Low in infection, RA, malignancy |
| Cytology | Malignancy |
| MC&S | Infection |
| Gram stain | Bacteria |
Algorithm
PLEURAL EFFUSION CONFIRMED
↓
┌────────────────────────────────────────────────────────┐
│ DIAGNOSTIC ASPIRATION │
│ (Ultrasound-guided) │
│ Send for: Protein, LDH, pH, glucose, MC&S, cytology │
└────────────────────────────────────────────────────────┘
↓
Transudate or Exudate?
↓ ↓
TRANSUDATE EXUDATE
Treat cause Investigate cause
(HF, cirrhosis) CT, consider thoracoscopy
↓
Is it infected? (pH less than 7.2)
↓ Yes → CHEST DRAIN
↓ No → Treat underlying cause
Chest Drain Indications
- Empyema (pus, organisms, pH less than 7.2)
- Large symptomatic effusion
- Haemothorax
- Recurrent malignant (consider indwelling catheter or pleurodesis)
- BTS Guideline for Pleural Disease. Thorax. 2010. PMID: 20696691
Last Reviewed: 2026-01-01 | MedVellum Editorial Team