Pathologic fluid accumulation in pleural space — transudate vs exudate by Light criteria.
Also known as: pleural effusion, transudate, exudate, empyema, hemothorax, chylothorax
Overview
Excess fluid in the pleural space resulting from imbalance between fluid production and absorption. Classified as transudate (systemic factors) or exudate (local pleural disease) using Light criteria.
Epidemiology
Estimated >1.5 million pleural effusions annually in the US. Heart failure is the most common cause overall; malignancy is the most common cause of unilateral exudative effusion in adults >50.
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Transudate: imbalance of hydrostatic and oncotic pressures with intact pleura — heart failure, cirrhosis, nephrotic syndrome. Exudate: increased pleural capillary permeability or impaired lymphatic drainage from inflammation, infection, or malignancy.
Clinical presentation
Symptoms
Dyspnea (proportional to size and rate of accumulation)
Pleuritic chest pain (more with exudates)
Cough (often dry)
Symptoms of underlying disease: fever (infection), weight loss (malignancy), orthopnea (HF)
Signs / physical exam
Decreased breath sounds, dullness to percussion, decreased tactile fremitus over effusion
Egophony at upper border (compressive atelectasis)
Pleural friction rub may precede or follow effusion
Hemothorax — Trauma; pleural fluid Hct >50% of serum Hct
Diagnostic workup
Diagnostic criteria
Light's criteria — exudate if ANY: pleural fluid protein / serum protein >0.5, pleural fluid LDH / serum LDH >0.6, or pleural fluid LDH > 2/3 upper limit of normal serum LDH. Otherwise transudate (consider serum-pleural albumin gradient >1.2 g/dL to reclassify if patient on diuretics).
Labs
Pleural fluid analysis (essential for new unexplained effusion):
Cell count and differential
Total protein, LDH, glucose, pH
Cytology (send large volume if malignancy suspected)
Gram stain and culture (aerobic, anaerobic, mycobacterial)
Triglycerides if chyle suspected; amylase if pancreatic/esophageal rupture; ADA if TB suspected
Serum: simultaneous total protein, LDH, albumin
BNP, CBC, BMP, LFTs to characterize underlying disease
Imaging
Upright PA/lateral CXR — blunting of costophrenic angle (≥200 mL); lateral decubitus or ultrasound identifies smaller effusions
Ultrasound — guides safe thoracentesis (REQUIRED per ATS); identifies loculations
Hemothorax: tube thoracostomy (28-32 Fr); urgent thoracotomy if initial output >1500 mL or ongoing >200 mL/h
Second-line / adjunct
Recurrent malignant pleural effusion: indwelling tunneled pleural catheter (PleurX) — outpatient drainage, may achieve auto-pleurodesis; alternative talc pleurodesis via chest tube or VATS
TB pleuritis: standard 4-drug RIPE therapy; effusions resolve without drainage in most
Refractory hepatic hydrothorax: TIPS or transplant
Chylothorax: low-fat MCT diet, octreotide, thoracic duct ligation if refractory
Complications
Trapped lung (lung fails to re-expand after drainage → ex-vacuo physiology)
Empyema, fibrothorax
Re-expansion pulmonary edema (drain ≤1-1.5 L per session)
Iatrogenic pneumothorax or hemothorax from thoracentesis (~5% — reduced with ultrasound guidance)
PANCE pearls
Light's criteria has high sensitivity for exudate but misclassifies ~25% of HF effusions on diuretics — use serum-pleural albumin gradient >1.2 g/dL to reclassify in that setting.
Always use ultrasound guidance for thoracentesis — reduces iatrogenic pneumothorax and dry tap rates substantially.
Glucose <60, pH <7.2, and elevated LDH point to empyema, malignancy, RA, or TB.
ADA >40 in a lymphocytic exudate strongly suggests TB pleuritis; pleural biopsy with granulomas is diagnostic.
Limit single-tap drainage to ~1.0-1.5 L (or until cough/chest discomfort) to avoid re-expansion pulmonary edema.
References
BTS 2023 — BTS Clinical Statement on Pleural Disease (Roberts et al., Thorax 2023)
MIST2 Trial — Intrapleural Use of Tissue Plasminogen Activator and DNase in Pleural Infection (Rahman et al., NEJM 2011)
Light Criteria — Pleural Effusions: The Diagnostic Separation of Transudates and Exudates (Light et al., Ann Intern Med 1972)
TIME-2 Trial — Effect of an Indwelling Pleural Catheter vs Chest Tube and Talc Pleurodesis for Relieving Dyspnea (Davies et al., JAMA 2012)
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