• Clinical science
  • Clinician

Pleural effusion


Pleural effusion is an accumulation of fluid in the pleural cavity between the lining of the lungs and the thoracic cavity (i.e., the visceral and parietal pleurae). The pleural fluid is called a transudate if it permeates (transudes) into the pleural cavity through the walls of intact pulmonary vessels. It is called an exudate if it escapes (exudes) into the pleural cavity through lesions in blood and lymph vessels, e.g., as caused by inflammation and tumors. The accumulation of transudate is typically due to increased hydrostatic pressure (e.g., in congestive heart failure) and/or decreased oncotic pressure (e.g., in cirrhosis or nephrotic syndrome). Since transudate is a filtrate, it is typically a clear fluid with a low protein and cell content. By contrast, the lesions responsible for the outflow of exudate allow larger molecules and even solid matter to pass into the pleural cavity. For this reason, exudate is a cloudy fluid with a high protein and cell content. The effusion follows gravity and, unless the patient is bedridden, collects in the lower margins of the pleural cavity. Percussion over the area of effusion generates a dull tone, and breath sounds are diminished or completely absent on auscultation. Chest x-ray and ultrasound are usually performed as first-line tests to diagnose pleural effusion, but chest CT is sometimes required (e.g., for very small effusions). Thoracentesis with pleural fluid analysis is required to establish the underlying diagnosis in most pleural effusions and can also serve a therapeutic role. Treatment should focus on correcting the underlying condition.


  • Definition: an excessive amount of fluid between pleural layers that impairs the expansion of the lungs

Transudative pleural effusion

Exudative pleural effusion
Common causes of pleural effusion [1][2]
Rare causes [1][2]
Light's Criteria

Pleural fluid protein:serum protein ratio

  • ≤ 0.5
  • > 0.5

Pleural fluid LDH:serum LDH ratio

  • ≤ 0.6
  • > 0.6

Pleural fluid LDH

  • < ⅔ the upper limit of normal serum LDH
  • > ⅔ the upper limit of normal serum LDH


Clinical features

Symptoms [2]

Physical exam findings

  • Inspection and palpation
  • Auscultation
  • Percussion: dullness over the area of effusion



  • Imaging is necessary to confirm the diagnosis
  • Consider diagnostic thoracentesis if the diagnosis is uncertain or management requires additional information (e.g., culture, cytology)
  • Consider invasive testing (e.g., bronchoscopy, VATS) if the diagnosis remains unclear

Imaging [6][2]

Chest x-ray [6][7]

Ultrasound [6][8]

  • Indications
  • Procedure: The imaging sequence is similar to the FAST protocol commonly used for trauma assessment.
  • Supportive findings: hypoechoic/anechoic structures in the lower margins of the pleural cavity (costodiaphragmatic recess)
    • Very sensitive: can detect fluid amounts as low as 20 mL
    • Hemothorax and empyema may appear heterogeneous.
    • Pleural fluid septations may be present.
    • Allows for detection of pleural thickening and pleural nodules

Chest CT [6][8]

  • Indications: gold standard but use is limited due to radiation and contrast exposure
    • Guiding placement of indwelling pleural catheters
    • Directed thoracentesis of a loculated effusion
    • Suspected parenchymal or pleural pathology
    • Better quantification of the amount of fluid (compared to CXR) is desired
  • Procedure: Chest CT without IV contrast is usually sufficient
    • With IV contrast: allows for the detection of underlying malignancy (e.g., metastases and/or primary tumor)
    • CT angiogram: helps to identify vascular pathology (e.g., dissection or AVM)
  • Supportive findings
    • Similar to CXR; can detect > 3–5 mL of fluid
    • Fluid density measurement can help differentiate pleural effusion from empyema and hemothorax [9]
    • Disease-specific signs: See pleural empyema.

Diagnostic thoracentesis [2][10]

Analysis of the pleural fluid (via thoracentesis) is usually required to definitively establish the underlying etiology but may not be necessary if there is already a clear diagnosis of an underlying condition (e.g., known CHF or connective tissue disease).


Indications [2][10]

Contraindications to thoracentesis

Pleural fluid studies to order

See tables below for analysis and interpretation.


See therapeutic thoracentesis.

Pleural fluid analysis [8][2][11]

Differentiating transudates from exudates (incl. Light's criteria) [12][13]

Primary pleural fluid analysis
Laboratory parameters Transudative effusion Exudative effusion
Light's criteria Pleural fluid protein:serum protein ratio
  • ≤ 0.5
  • > 0.5
Pleural fluid LDH:serum LDH ratio
  • ≤ 0.6
  • > 0.6
Pleural fluid LDH
  • < ⅔ the upper limit of normal serum LDH
Pleural fluid cholesterol
  • < 55 mg/dL
  • > 55 mg/dL
Pleural fluid LDH
  • < 200 U/L
  • > 200 U/L
Pleural fluid cholesterol:serum cholesterol ratio
  • < 0.3
  • > 0.3
Adjunctive pleural fluid analysis [2][8][11]
Pleural fluid parameter Suggests transudative effusion Suggests exudative effusion
Physical appearance
  • Clear fluid
  • Does not froth or form clots
  • Cloudy or straw-colored fluid (may be hemorrhagic in rare cases)
  • Froths when shaken and forms clots when left standing
Specific gravity
  • ≤ 1.016
  • > 1.016


Normal pH ∼ 7.6

  • 7.4–7.55
  • < 7.3–7.45
  • ≥ 60 mg/dL
  • < 60 mg/dL
Total protein gradient
  • > 3.1 g/dL lower than serum total protein
  • < 3.1 g/dL lower than serum total protein
Albumin gradient
  • > 1.2 g/dL lower than serum albumin
  • < 1.2 g/dL lower than serum albumin

Narrowing the differential diagnosis of exudative effusions

Pleural fluid parameter Associated conditions [10][1][14]
Cell count and differential WBC count > 10,000 cells/mm3
Neutrophils > 50% of total leukocytes
Lymphocytes > 50% of total leukocytes
RBC count > 5,000 cells/μL
Hematocrit > 0.5 × peripheral hematocrit
pH < 7.2 [14]

Glucose < 60 mg/dL

Positive Gram stain or culture

Adenosine deaminase > 50 mcg/L

Positive AFB smear microscopy

  • Tuberculous effusion
Abnormal cytology
Amylase > 200 mcg/dL
Positive rheumatoid factor, ANA
Lipids Triglycerides > 110 mg/dL

Total cholesterol > 200 mg/dL

Total cholesterol:triglyceride ratio > 1

Cholesterol crystals

Total cholesterol 55–200 mg/dL

Chylomicrons and fat-soluble vitamins

Appearance Cloudy, milky

Transudate is usually clear, has a decreased cell count, and has low levels of protein, albumin, and LDH. Exudate typically appears cloudy, has an increased cell count, and has high levels of protein, albumin, and LDH.

Think of MEAT to memorize causes of pulmonary effusion associated with low glucose levels: Malignancy, Empyema, Arthritis (rheumatoid pleurisy), Tuberculosis.

Pleural fluid with a bloody appearance suggests a malignant etiology or hemothorax!

Additional serum laboratory studies and invasive diagnostic tests [12]


Approach [10][12][14]

Treat the underlying cause [12]

Therapeutic thoracentesis [10]

The goal of a therapeutic thoracentesis is to remove fluid (especially in exudate because of increased risk of infection). Removal of 400–500 mL of fluid is usually sufficient to relieve symptoms (e.g., dyspnea).



See contraindications to thoracentesis.


Therapeutic thoracentesis should be halted if patients develop chest discomfort, cough, or hypoxia, which could represent re-expansion pulmonary edema.

Indwelling pleural catheter [21][22][23][24]

  • Goal: recurrent pleural fluid removal without repeated puncture
  • Indication: rapidly reaccumulating pleural effusions (e.g., malignant effusions ) [23][24]
  • Contraindications: See contraindications to thoracentesis.
  • Procedure [25]
    • A pleural catheter is inserted under the guidance of interventional radiology.
    • Similar to thoracentesis, except that the catheter is tunneled into the skin and can remain in situ for months.
  • Complications [25][26]

Surgical procedures [12]

Consultation with a thoracic surgeon and/or chest physician is recommended.

Tube thoracostomy

  • Indications
    • For recurrent pleural effusion or urgent drainage of infected and/or loculated effusions [27][28]
    • Drainage of high-viscosity fluid that is likely to clog [29][30]
  • Procedure: See tube thoracostomy.

Video-assisted thoracoscopic surgery (VATS)

Pleurodesis [22][23]

  • Definition: chemical or surgical obliteration of the pleural space
  • Indication
  • Contraindications [21]
    • Predicted survival < 3 months
    • Trapped lung
  • Procedure
    • After draining the pleural effusion, a substance (e.g., talc) is introduced into the pleural cavity.
    • This induces an inflammatory reaction that causes the pleural layers to bind together.
    • Alternatively, thoracoscopic pleurodesis with partial resection of the pleural layers may be performed.
  • Complication: fibrothorax

A chest x-ray should be performed after each of these procedures to rule out iatrogenic pneumothorax!

Acute management checklist

Subtypes and variants

Pleural fluid analysis is necessary in almost all cases to distinguish between the various subtypes of pleural effusion. Treatment depends on the underlying cause. Subtypes of pleural effusion include the following:

Parapneumonic effusion

Definition [36][2]

  • Accumulation of exudative fluid in the pleural cavity in response to pneumonia
    • Uncomplicated: without direct bacterial invasion
    • Complicated: extension of bacterial infection into the pleural space

Distinguishing features [36][6][14][10]

Treatment [10][37][38][39][28]

Pleural empyema

Definition [36]

Etiology [36][14]

Classification [40][41]

  • Stage I (exudative): accumulation of fluid and pus
  • Stage II (fibrinopurulent): aggregation of fibrin deposits that form septations and pockets
  • Stage III (organizing): formation of thick fibrous peel on pleural surface that restricts lung movement

Distinguishing features [36]

Treatment of pleural empyema

Empiric antibiotic therapy for pleural infection [37][38][39][28]

All patients should receive empiric antibiotics adjusted to their needs, local resistance patterns, and institutional guidelines.

Definitive treatment [40]

  • Stage I
    • Chest tube (thoracostomy) to remove empyema fluid
    • Consider intrapleural administration of fibrinolytic agents.
  • Stage II or mixed stage II/III
  • Stage III

Nontraumatic hemothorax

Definition [43][34]

  • Spontaneous or nontraumatic accumulation of blood in the pleural cavity

Etiology [34]

Distinguishing features

Treatment of nontraumatic hemothorax [43][34]

  • Stabilize the patient.
  • Identify and treat the underlying cause.
  • Consult surgery or interventional radiology for bleeding from tumors or vascular pathology.

A hemothorax, however small, must always be drained because blood in the pleural cavity will clot if not evacuated, resulting in a trapped lung or an empyema.

Malignant pleural effusion

Description [45]

  • Definition: accumulation of exudative fluid and malignant cells in the pleural cavity
  • Pathophysiology: cancer-related barrier dysfunction of the capillary walls → increased permeation of plasma protein, blood cells, and tumor cells

Etiology [45]

Distinguishing features [45]

Treatment [46]


Definition [8]

Etiology [8]

Distinguishing features [8]

Treatment of chylothorax [47]

  • First-line conservative treatment of chylothorax
  • Surgical treatment: via thoracotomy, thoracoscopy, or VATS
    • Indications
      • Unsuccessful conservative treatment
      • Clinical deterioration (e.g., nutritional or metabolic)
      • Chyle drainage
        • > 1000–1500 mL/day
        • Up to 1000 mL/day for ≥ 5 days
        • Unchanged over 1–2 weeks
      • Persistent chyle leak: > 100 mL/day for > 2 weeks
    • Procedures
  • Interventional radiology
    • Indications: an alternative for patients who cannot tolerate operative procedures and for whom the treatment is anatomically feasible
    • Procedures
      • TIPS procedure (for hepatic chylothorax)
      • Embolization or disruption of the lymph ducts
      • Percutaneous repair of the thoracic ducts


Definition [8][48]

Etiology [8][48]

Distinguishing features [8][48]

In contrast to chylothorax, a pseudochylothorax is characterized by high cholesterol and low triglyceride levels in the pleural fluid. The presence of cholesterol crystals may also help to differentiate a pseudochylothorax from a chylothorax.


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last updated 11/23/2020
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