• 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 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.


Transudative effusion

Exudative effusion
  • Capillary permeability (e.g., due to inflammation)
Common causes


Clinical features

Symptoms [5]

Physical exam findings

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


Subtypes and variants

Parapneumonic effusion and pleural empyema [6][7][8][9]

Chylothorax [14]

Pseudochylothorax [15][14]

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 differentiate a pseudochylothorax from a chylothorax.

Hemothorax [16]

  • Description: accumulation of blood in the pleural cavity
  • Etiology
  • Clinical features
    • See “Clinical features” above.
    • If significant blood loss: hypotension, tachycardia
    • If due to trauma
  • Diagnostics
  • Treatment
    • Chest tube (thoracostomy) with blood evacuation
    • Surgical exploration and repair in the case of
      • Massive hemothorax (> 1000 mL)
      • Continuing bleeding (150–200 mL/hr for 2–4 hours)
      • Sustained hemodynamic instability requiring repeated transfusions

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 [17]

  • Description: accumulation of malignant cells in the pleural cavity
  • Etiology: caused by either direct invasion of the pleural space or distant metastases
  • 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
  • Diagnostics
    • Chest x-ray
      • See “Diagnostics” below.
      • Underlying primary lung cancer may be visualized.
    • Chest ultrasound
      • Allows for the detection of small amounts of fluid
      • Can be used to assess pleural thickening and presence of pleural nodules
    • Chest CT: detection of fluid, metastases, and/or primary tumor
    • Pleural fluid analysis
      • Cell-rich exudative effusion
      • pH usually low (< 7.3)
      • Glucose usually low, but higher than in empyema
      • LDH usually high
      • Cytology [14]
        • Abnormal morphological features allow for the identification of malignant cells ; (e.g., pronounced nucleoli, cells with multiple nuclei, numerous figures of mitosis).
        • Immunohistochemistry can help to differentiate between types of malignant cells.
  • Treatment [18]



Imaging can confirm a pleural effusion, but analysis of the pleural fluid (via thoracentesis) is usually required to establish the underlying etiology.


Chest x-ray

  • Very small pleural effusions (< 300 mL) may not be visible on a chest x-ray but can be detected on ultrasound.
  • Findings [19]
    • Typically unilateral blunting of the costophrenic angle
    • Homogeneous density with meniscus-shaped margin
    • Large effusion
      • Complete opacification of the lung
      • Mediastinal shift
      • Tracheal deviation away from the effusion (space-occupying lesion)
  • Lateral decubitus view: can help determine whether the fluid is encapsulated (loculated) or free [20][21]

Ultrasound [14][19]

  • Very sensitive: can detect fluid amounts as low as 20 mL
  • Findings: hypoechoic/anechoic structures in the lower margins of the pleural cavity (costodiaphragmatic recess)
  • High sensitivity for pleural fluid septations
  • Allows for detection of pleural thickening and pleural nodules
  • Commonly used for planning thoracentesis

Chest CT [14]

  • More sensitive than CXR and ultrasound in identifying small pleural effusions but not required for diagnosis
  • Measurement of fluid density on CT scan may provide a clue to the underlying etiology (e.g., hyperattenuation of blood in hemothorax).
  • Consider IV contrast if there is a concern for underlying malignancy (e.g., heavy asbestos exposure history).


  • Aspiration of fluid from the pleural space for diagnostic (e.g., transudate vs. exudate) and/or therapeutic purposes
  • Indications
    • 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
Pleural fluid analysis [22][23][24][14]
Transudative effusion Exudative effusion
Physical appearance
  • Does not froth or form clots
  • Cloudy or straw-colored fluid (may rarely be hemorrhagic)
  • Froths when shook and forms clots when left standing
Specific gravity
  • ≤ 1.016
  • > 1.016


Normal pH ∼ 7.6

  • 7.4–7.55
  • ≥ 60 mg/dL
  • < 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
  • Very high LDH levels (e.g., > 1000 IU/L) suggest empyema, malignancy, or rheumatoid effusion.

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 MEAT to memorize causes of pulmonary effusion with decreased glucose content: M = Malignancy, E = Empyema, A = Arthritis (rheumatoid pleurisy), T = Tuberculosis!

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



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