Pleural effusions may be classified as being either a transudate or exudate according to the protein concentration.

Transudate (< 30g/L protein)
  • Heart failure (most common transudate cause)
  • Hypoalbuminaemia (liver disease, nephrotic syndrome, malabsorption)
  • Hypothyroidism
  • Meigs' syndrome

Exudate (> 30g/L protein)
  • Infection: pneumonia (most common exudate cause), TB, subphrenic abscess
  • Connective tissue disease: RA, SLE
  • Neoplasia: lung cancer, mesothelioma, metastases
  • Pancreatitis
  • Pulmonary embolism
  • Dressler's syndrome
  • Yellow nail syndrome

Clinical features

  • Dyspnoea, non-productive cough or chest pain are possible presenting symptoms
  • Classic examination findings include dullness to percussion, reduced breath sounds and reduced chest expansion


The British Thoracic Society (BTS) produced guidelines in 2010 covering the investigation of patients with a pleural effusion.

  • Posterioranterior (PA) chest x-rays should be performed in all patients
  • Ultrasound is recommended: it increases the likelihood of successful pleural aspiration and is sensitive for detecting pleural fluid septations
  • Contrast CT is now increasingly performed to investigate the underlying cause, particularly for exudative effusions

Pleural aspiration
  • As above, ultrasound is recommended to reduce the complication rate
  • A 21G needle and 50ml syringe should be used
  • Fluid should be sent for pH, protein, lactate dehydrogenase (LDH), cytology and microbiology

Light's criteria was developed in 1972 to help distinguish between a transudate and an exudate. The BTS recommend using the criteria for borderline cases:
  • Exudates have a protein level of >30 g/L, transudates have a protein level of <30 g/L
  • If the protein level is between 25-35 g/L, Light's criteria should be applied. An exudate is likely if at least one of the following criteria are met:
  • Pleural fluid protein divided by serum protein >0.5
  • Pleural fluid LDH divided by serum LDH >0.6
  • Pleural fluid LDH more than two-thirds the upper limits of normal serum LDH

Other characteristic pleural fluid findings:
  • Low glucose: rheumatoid arthritis, tuberculosis
  • Raised amylase: pancreatitis, oesophageal perforation
  • Heavy blood staining: mesothelioma, pulmonary embolism, tuberculosis

Pleural infection

All patients with a pleural effusion in association with sepsis or a pneumonic illness require diagnostic pleural fluid sampling
  • If the fluid is purulent or turbid/cloudy a chest tube should be placed to allow drainage
  • If the fluid is clear but the pH is less than 7.2 in patients with suspected pleural infection a chest tube should be placed


Management of recurrent pleural effusion

Options for managing patients with recurrent pleural effusions include:
  • Recurrent aspiration
  • Pleurodesis
  • Indwelling pleural catheter
  • Drug management to alleviate symptoms e.g. opioids to relieve dyspnoea