• Clinical science

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 essentially 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., as in congestive heart failure) and/or decreased oncotic pressure (e.g., as in cirrhosis or nephrotic syndrome). Since transudate is a filtrate, it is typically a clear fluid with 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. Over the area of effusion, percussion generates a dull tone, and breath sounds are diminished or completely absent on auscultation. Ultrasound is the method of choice for confirming the diagnosis. To establish etiology in cases of first-time or idiopathic pleural effusion, a biopsy for microbiological, cytological, and clinical chemistry testing is recommended. Only large pleural effusions lead to shortness of breath and can be drained via a pleural tap (thoracentesis) if necessary. As a general rule, treatment should focus on the underlying condition.



  • Capillary permeability
Common causes


Clinical features


Subtypes and variants

Pleural empyema

  • Definition: accumulation of pus in the pleural cavity
  • Etiology: most commonly occurs secondary to an infected parapneumonic pleural effusion; . Less common causes include untreated hemothorax that becomes infected, ruptured lung abscess, esophageal tear, or thoracic trauma (e.g., thoracic surgery)
  • Clinical features
  • Diagnosis
  • Treatment



Malignant pleural effusion

  • Pathophysiology: increased permeation of plasma protein, blood cells, and tumor cells into the pleural space caused by a cancer-related barrier dysfunction of the capillary walls
  • Diagnosis: Thoracocentesis with pleural fluid analysis
    • Cell-rich exudate
    • Criteria for malignancy: pronounced nucleoli, cells with multiple nuclei, numerous figures of mitosis



Physical examination

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


Chest x-ray

  • Very small PE (< 300 mL) may not be visible on a chest x-ray, but can be detected on ultrasound.
  • Findings: often bilateral and symmetric
    • Blunting of costophrenic angle
    • Homogeneous density with meniscal-shaped margin.
    • Large effusion: complete shadowing of the lung with mediastinal shift and tracheal deviation away from the effusion
  • Lateral decubitus view: demonstrates whether fluid is encapsulated (loculated) or free


Chest CT

  • Even more sensitive than ultrasound


  • Aim: removal of fluid from the pleural space for diagnostic (e.g., transudate vs. exudate) and/or therapeutic purposes; should be preceded by imaging
  • Indication:
    • Any new unilateral effusion > 1 cm on x-ray in an undiagnosed patient
    • History of malignant tumor with effusion > 1 cm on x-ray
    • Large effusion with dyspnea and/or cardiac decompensation
    • Pneumonia with parapneumonic effusion > 5 cm on x-ray
    • Heart failure in conjunction with atypical findings (e.g., pleuritic chest pain, fever, unilateral effusion)
    • Suspected transudative bilateral effusions with atypical features (e.g., fever, pleuritic chest pain, effusions of disparate size)
  • Technique:
    • Ultrasound-assisted (if possible) dorsal puncture with the patient sitting upright
    • Puncture site: 1–2 ICS beneath the upper margin of the largest pocket of fluid, but not below the 8th ICS
    • No more than 1.5 L should be drained with each puncture, as otherwise a re-expansion edema may occur.
    • One cannula for microbiological analysis → bacterial cultures, Gram, or Ziehl-Neelsen staining (optional)
    • One cannula for clinical chemical analysis total protein content, LDH
    • One cannula for pathological analysis → cytological smear to rule out malignant effusion (see "Pathology" below)
Transudate Exudate
Physical appearance Does not froth or form clots Straw-colored fluid (may rarely be hemorrhagic), which froths on shaking and forms clots on standing still
Specific gravity ≤ 1.016 > 1.016
Glucose ≥ 60 mg/dL
Cholesterol < 60 mg/dL

≥ 60 mg/dL

Total protein ≤ 30 g/L > 30 g/L
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 (lactate dehydrogenase) < ⅔ the upper limit of normal serum LDH Pleural fluid LDH > ⅔ the upper limit of normal serum LDH

Pleural fluid with a bloody appearance suggests a malignant etiology!



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