Exudate vs Transudate
Overview
Distinguishing an exudate from a transudate in any body cavity (pleural, peritoneal, pericardial) is fundamental. Transudates arise from altered hydrostatic or oncotic forces; exudates arise from active inflam-mation, infection, or malignancy. Different aetiologies, different management.
Transudate
- Mechanism: Increased hydrostatic pressure (CHF, fluid overload) OR decreased plasma oncotic pressure (hypoalbuminaemia in nephrotic syndrome, liver failure).
- Characteristics: Low protein (<30 g/L), low LDH, low cell count, clear appearance.
- Examples: Heart failure, cirrhosis (SAAG >=1.1), nephrotic syndrome, hypothyroidism.
Exudate
- Mechanism: Increased vascular permeability from inflammation, infection, or malignancy affecting the pleura/peritoneum.
- Characteristics: High protein (>30 g/L), elevated LDH, elevated cell count, often turbid or bloody.
- Examples: Parapneumonic effusion, empyema, malignancy, TB, pancreatitis, PE.
Light's Criteria (Pleural Fluid)
An effusion is an exudate if ANY ONE of the following is met:
- Pleural fluid protein / Serum protein ratio > 0.5
- Pleural fluid LDH / Serum LDH ratio > 0.6
- Pleural fluid LDH > 2/3 the upper limit of normal serum LDH
WarningLight's Criteria Misclassification
Light's criteria misclassifies up to 25% of CHF transudates as exudates due to diuretic therapy reducing pleural protein concentration. In these cases: (Serum Albumin - Pleural Fluid Albumin >1.2 g/dL) re-classifies them as transudates.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Imaging cannot reliably distinguish exudate from transudate. Ultrasound can detect complex echogenic fluid (suggesting exudate/haemorrhage) vs. completely anechoic fluid (suggesting transudate). CT can demonstrate peritoneal enhancement or nodularity (suggesting malignancy/infection). Ultimately, thoracocentesis/paracentesis with fluid analysis is required.