Residency · Residency · Internal Medicine

Pleural Effusions: Diagnostic and Therapeutic Approach

Anatomy and Physiology

The pleural space normally contains just 5 to 15 mL of fluid, maintained in equilibrium by Starling forces acting across the parietal and visceral pleura. Fluid enters the space through parietal pleural capillaries, which are supplied by the systemic circulation, and is reabsorbed primarily via parietal pleural lymphatics with a drainage capacity of approximately 700 mL per day. A pleural effusion develops when fluid formation exceeds absorption, which can result from increased hydrostatic pressure, decreased oncotic pressure, increased capillary permeability, impaired lymphatic drainage, or transdiaphragmatic passage of ascitic fluid.

Classification: Transudative vs. Exudative

Light's Criteria (Sensitivity ~98%, Specificity ~77%)

An effusion is classified as EXUDATIVE if it meets ANY ONE of the following:

Specifically, pleural fluid protein divided by serum protein exceeds 0.5, pleural fluid LDH divided by serum LDH exceeds 0.6, or pleural fluid LDH exceeds two-thirds the upper limit of normal for serum LDH. Light's criteria misclassify approximately 25% of transudates as exudates, particularly in patients receiving diuretic therapy. When a transudate is clinically suspected but Light's criteria suggest an exudate, the serum-to-pleural fluid albumin gradient should be checked. A gradient greater than 1.2 g/dL supports a transudative process despite meeting Light's criteria.

Common Transudative Causes

Heart failure is the most common overall cause of pleural effusion in developed countries. Hepatic hydrothorax occurs when ascitic fluid passes transdiaphragmatically into the pleural space, appearing on the right side in roughly 70% of cases. Other transudative causes include nephrotic syndrome, peritoneal dialysis, atelectasis (ex vacuo effusion), myxedema, and constrictive pericarditis.

Common Exudative Causes

Parapneumonic effusion and empyema represent the most common exudative cause. Malignancy, particularly from lung cancer, breast cancer, and lymphoma, along with mesothelioma, is another major category. Pulmonary embolism produces exudative effusions in about 80% of cases and transudative in 20%. Tuberculosis, autoimmune diseases such as rheumatoid arthritis and lupus pleuritis, and pancreatitis (characteristically left-sided with elevated amylase) are additional important causes. Chylothorax is identified by triglycerides exceeding 110 mg/dL, while hemothorax is defined by a pleural hematocrit greater than 50% of the peripheral value. Drug-induced effusions can occur with nitrofurantoin, amiodarone, methotrexate, and checkpoint inhibitors. Post-cardiac surgery effusions may represent post-pericardiotomy syndrome.

Diagnostic Approach

When to Perform Thoracentesis

Thoracentesis should be performed for any new clinically significant effusion measuring greater than 10 mm on lateral decubitus imaging or ultrasound, and for any known transudative effusion with atypical features such as unilateral presentation, fever, or pleuritic pain. Observation without thoracentesis is reasonable for bilateral small effusions in a patient with known heart failure who is responding to diuretics.

Thoracentesis Technique

Ultrasound guidance is the standard of care for thoracentesis, reducing pneumothorax risk from 10-18% with landmark technique to less than 2%. The insertion site should be one to two intercostal spaces below the top of the effusion, with the needle directed above the rib to avoid the neurovascular bundle that runs along the inferior rib margin. For diagnostic purposes, 50 to 60 mL is sufficient, while therapeutic drainage may continue until dry or up to 1 to 1.5 liters per session to minimize the risk of re-expansion pulmonary edema, though recent evidence suggests this threshold may be conservative. A post-procedure chest X-ray is not routinely needed if the procedure was ultrasound-guided and the patient has no symptoms of pneumothorax.

Pleural Fluid Analysis -- Standard Studies

The appearance of the fluid itself provides initial clues: straw-colored fluid suggests a transudate, cloudy or turbid fluid indicates an exudate or empyema, bloody fluid raises concern for malignancy or hemothorax, and milky fluid points toward chylothorax. Cell count and differential are informative: neutrophilic predominance suggests pneumonia, pulmonary embolism, pancreatitis, or early tuberculosis, while lymphocytic predominance greater than 80% points toward tuberculosis, malignancy, sarcoidosis, post-CABG effusion, or rheumatoid disease. Eosinophilic effusions (greater than 10%) are associated with air or blood in the pleural space, drug reactions, parasitic infections, asbestos exposure, or less commonly malignancy. Protein and LDH are used for Light's criteria. Glucose below 30 mg/dL is characteristic of empyema, rheumatoid pleurisy, esophageal rupture, or malignancy. A pH below 7.20 in a parapneumonic effusion indicates a complicated effusion requiring drainage, though pH is unnecessary in frank empyema where drainage is already indicated. Cytology has approximately 60% sensitivity for malignancy on a first sample, increasing to about 75% with a second sample. Cultures and Gram stain are obtained for suspected infections.

Additional Studies Based on Clinical Suspicion

Adenosine deaminase (ADA) at 40 U/L or above is highly suggestive of tuberculous pleurisy, with sensitivity and specificity both around 90% in high-prevalence areas. Triglycerides above 110 mg/dL confirm chylothorax while levels below 50 effectively rule it out. Elevated amylase suggests pancreatitis, esophageal rupture, or malignancy. NT-proBNP above 1500 pg/mL supports heart failure as the cause. A pleural-to-blood hematocrit ratio exceeding 0.5 defines hemothorax.

Parapneumonic Effusions and Empyema

Classification

Simple parapneumonic effusions are reactive and sterile, resolving with antibiotics alone. Complicated parapneumonic effusions indicate an infected pleural space, characterized by pH below 7.20, glucose below 60, LDH above 1000, or positive Gram stain and culture, and they require drainage. Empyema is defined as frank pus in the pleural space or positive Gram stain and culture, and it always requires drainage.

Indications for Pleural Drainage (Tube Thoracostomy)

Drainage is indicated for frank pus, positive Gram stain or culture, pH below 7.20, glucose below 60 mg/dL, loculated effusions, or large effusions occupying more than 50% of the hemithorax.

Management

Antibiotics should cover pneumonia pathogens appropriate to the setting (community-acquired versus healthcare-associated), with anaerobic coverage added if aspiration is suspected. A small-bore chest tube (10-14 Fr) is adequate for most parapneumonic effusions, while large-bore tubes are reserved for empyema with thick pus. Intrapleural fibrinolytic therapy with tPA (10 mg) plus DNase (5 mg) administered twice daily for three days is supported by the MIST2 trial, which demonstrated that the combination improved fluid drainage and reduced surgical referral. Notably, neither agent alone was effective; the combination is essential. Surgical options include video-assisted thoracoscopic surgery (VATS) for failed medical therapy and open thoracotomy with decortication for chronic empyema with trapped lung.

Malignant Pleural Effusions

Diagnosis

Cytology has about 60% sensitivity per sample, so repeat thoracentesis or pleural biopsy should be pursued if the first sample is negative. Image-guided or thoracoscopic pleural biopsy achieves sensitivity above 90% for mesothelioma and tuberculosis. Biomarkers such as CEA and mesothelin (for mesothelioma) may be helpful. Medical thoracoscopy or pleuroscopy allows direct visualization and biopsy with high diagnostic yield.

Management

The focus is symptom palliation through dyspnea relief rather than cure. Therapeutic thoracentesis is appropriate for patients with short life expectancy or infrequent reaccumulation. An indwelling pleural catheter (IPC) is a tunneled catheter enabling outpatient drainage that achieves spontaneous pleurodesis in 40-70% of patients over time and is preferred for trapped lung where pleurodesis will not work. Chemical pleurodesis with talc, the most effective agent, can be delivered via chest tube as slurry or via thoracoscopy as poudrage, but requires lung re-expansion for success, making trapped lung a contraindication. The TIME2 trial demonstrated that IPC and talc pleurodesis were equally effective for dyspnea relief in malignant effusions. In chemo-sensitive tumors such as lymphoma and small cell lung cancer, systemic therapy may resolve the effusion.

Hepatic Hydrothorax

Hepatic hydrothorax results from transdiaphragmatic passage of ascitic fluid through small diaphragmatic defects. It is right-sided in about 70% of cases, left-sided in 15%, and bilateral in 15%. Management mirrors that of ascites: sodium restriction, diuretics, and TIPS for refractory cases. Thoracentesis provides symptomatic relief, but chest tube placement is generally avoided due to the risk of persistent drainage, protein and electrolyte depletion, and infection. Pleurodesis has limited success because of ongoing ascites formation.

Chylothorax

Chylothorax results from disruption of the thoracic duct leading to chyle leakage. Causes include trauma or surgery (especially thoracic and esophageal), lymphoma, lymphangioleiomyomatosis, and sarcoidosis. Diagnosis is confirmed by milky fluid with triglycerides above 110 mg/dL and chylomicrons on lipoprotein analysis. Management begins conservatively with NPO status, octreotide, and a medium-chain triglyceride diet to reduce lymphatic flow, with thoracic duct ligation or embolization reserved for refractory cases.

<image> A diagnostic algorithm flowchart for pleural effusions. Start with "New Pleural Effusion on Imaging" leading to thoracentesis decision criteria. Branch into transudate (Light's criteria not met) vs. exudate (Light's criteria met). The transudate arm lists common causes with a note about albumin gradient correction for diuretic use. The exudate arm branches into infectious (parapneumonic/empyema), malignant, and other categories with specific additional tests for each. Include a sidebar showing Light's criteria values. </image>

<image> A medical illustration showing proper thoracentesis technique with ultrasound guidance. Show the patient in seated upright position, the ultrasound probe placement identifying the effusion and diaphragm, and the needle insertion site above the rib to avoid the neurovascular bundle. Include a magnified cross-section of the intercostal space showing the vein, artery, and nerve running along the inferior rib margin. Label the skin, subcutaneous tissue, intercostal muscles, parietal pleura, and pleural fluid. </image>

<image> A comparison infographic of management options for malignant pleural effusion. Three columns showing: (1) Repeated therapeutic thoracentesis -- best for limited life expectancy, (2) Indwelling pleural catheter -- best for trapped lung or outpatient management, and (3) Chemical pleurodesis with talc -- best for expandable lung and expected survival >3 months. Include success rates, advantages, disadvantages, and key trial evidence (TIME2 trial) for each approach. </image>

Clinical Pearls

Ultrasound-guided thoracentesis is the standard of care because landmark-based technique carries unacceptably higher pneumothorax rates. Light's criteria are highly sensitive but can misclassify transudates, especially in patients on diuretics; the serum-pleural albumin gradient greater than 1.2 g/dL serves as a correction factor. A pleural fluid pH below 7.20 in a parapneumonic effusion mandates drainage and represents the key decision point; importantly, pH should be sent in a heparinized blood gas syringe rather than a standard lab tube. In tuberculous pleuritis, the pleural fluid smear is often negative (only 5-20% positive), making ADA at or above 40 U/L and lymphocytic predominance the key diagnostic clues. Cytology has only about 60% sensitivity per sample for malignancy, so a negative result does not rule out a malignant effusion, and repeat thoracentesis or pleural biopsy should be considered. Hepatic hydrothorax can occur without clinically detectable ascites because small diaphragmatic defects allow preferential fluid flow into the pleural space. In any unexplained exudative effusion, pulmonary embolism should always be considered, as it can cause either transudative or exudative effusions.

References

• Light RW. Pleural Effusions. Medical Clinics of North America. 2011.
• Light RW, et al. Pleural Effusions: The Diagnostic Separation of Transudates and Exudates. Annals of Internal Medicine. 1972.
• Rahman NM, et al. MIST2 Trial: Intrapleural Use of tPA and DNase in Pleural Infection. NEJM. 2011.
• Davies HE, et al. TIME2 Trial: Management of Malignant Pleural Effusions. JAMA. 2012.
• Feller-Kopman D, et al. Management of Malignant Pleural Effusions: ATS/STS/STR Clinical Practice Guideline. Am J Respir Crit Care Med. 2018.
• Porcel JM, et al. Pleural Fluid Tests for the Diagnosis of Tuberculous Pleurisy. Current Opinion in Pulmonary Medicine. 2016.
• Havelock T, et al. BTS Guideline for Pleural Procedures. Thorax. 2010.

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