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Pleural effusion

Last updated: September 3, 2021

Summarytoggle arrow icon

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 vs exudative pleural effusion [1]

Transudative pleural effusion

Exudative pleural effusion
Common causes of pleural effusion [2][3]
Rare causes [2][3]
Light 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

Symptoms [3]

Physical exam findings

  • Inspection and palpation
  • Auscultation
    • Faint or absent breath sounds over the area of effusion
    • Pleural friction rub (squeaking sound of inflamed pleural layers rubbing together during inspiration and expiration)
  • 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 [3][4]

Chest x-ray [4][5]

Ultrasound [4][7]

Chest CT [4][7]

  • Indications: gold standard but use is limited due to radiation and contrast exposure
  • Procedure: chest CT without IV contrast is usually sufficient
  • Supportive findings
    • Can detect > 3–5 mL of fluid
    • Fluid density measurement can help differentiate pleural effusion from empyema and hemothorax [8]
    • Disease-specific signs: See “Pleural empyema.”

Diagnostic thoracentesis [3][9]

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).

Pleural fluid studies to order

See tables below for analysis and interpretation.


Pleural fluid analysis [3][7][10]

Differentiating transudates from exudates (incl. Light criteria) [6][11]

Primary pleural fluid analysis
Laboratory parameters Transudative effusion Exudative effusion
Light 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
  • < 45 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 [3][7][10]
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

Differential diagnosis of exudative effusions
Pleural fluid parameter Associated conditions [2][9][12]
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 [12]

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
  • Chylothorax
  • Tuberculous effusion
  • Accidental intrathoracic leak of TPN solution

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.

MEAT has low glucose: Malignancy, Empyema, Arthritis (rheumatoid pleurisy), and Tuberculosis are causes of pulmonary effusion associated with low glucose levels.

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

Additional serum laboratory studies and invasive diagnostic tests [6]

Approach [6][9][12]

Treat the underlying cause [6]

Therapeutic thoracentesis [9]

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).




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

Indwelling pleural catheter [19][20][21][22]

Surgical procedures [6]

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

Tube thoracostomy

  • Indications
    • Pleural effusion in combination with significant cardiac and/or respiratory decompensation [25]
    • For recurrent pleural effusion or urgent drainage of infected and/or loculated effusions [26][27]
    • Drainage of high-viscosity fluid that is likely to clog [28][29]
  • Procedure: See “Tube thoracostomy.”

Video-assisted thoracoscopic surgery (VATS)

Pleurodesis [20][21]

  • Definition: chemical or surgical obliteration of the pleural space
  • Indication
  • Contraindications [19]
  • 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.

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:

Definition [3][35]

Distinguishing features [4][9][12][35]

Treatment [9][27][36][37][38]

Definition [35]

Etiology [12][35]

Classification [39][40]

  • 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 [35]

Treatment of pleural empyema

Empiric antibiotic therapy for pleural infection [27][36][37][38]

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

Definitive treatment [39]

Overview of parapneumonic effusion and empyema
Uncomplicated parapneumonic effusion Complicated parapneumonic effusion Pleural empyema
  • Exudative effusion (without direct bacterial invasion)
  • Exudative effusion (with bacterial invasion)
Clinical features


Pleural fluid analysis
  • pH > 7.2
  • Glucose: normal/low
  • LDH
  • Appearance: slightly cloudy
  • pH < 7.2
  • Glucose: low
  • LDH
  • Appearance: cloudy
Gram stain and blood culture [38][39]
  • Negative
  • May be positive

Definition [33][42]

  • Spontaneous or nontraumatic accumulation of blood in the pleural cavity

Etiology [33]

Distinguishing features

Treatment of nontraumatic hemothorax [33][42]

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.

Description [44]

Etiology [44]

Distinguishing features [44]

Treatment [45]

Definition [7]

Etiology [7]

Distinguishing features [7]

Treatment of chylothorax [46]

  • 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
        • > 1,000–1,500 mL/day
        • Up to 1,000 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 [7][47]

Etiology [7][47]

Distinguishing features [7][47]

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.


Etiology [48][49]

Clinical features

Diagnosis [50]

Differential diagnosis of pleuritic chest pain [50]

Treatment [48]

  • Analgesia: NSAIDs (first-line) can be used for relief of symptoms
  • Treat underlying cause accordingly.
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