Pleural effusion

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Abnormal fluid in the pleural cavity. Sometimes confused with pulmonary oedema, which is fluid in the lungs, rather than outside them.


There are a variety of types:


  • Hydrothorax - water in the cavity, usually caused by ascites leaking in from the abdominal cavity.


  • Haemothorax - blood in the cavity, usually as a result of trauma.
  • Chylothorax - chyle(lymph fluid) in the cavity, usually as a result of lymphoma.
  • Empyema - pus in the cavity, usually as a result of infection.

According to Light's criteria (Light, et al. 1972), a pleural effusion is likely exudative if at least one of the following exists:-

  • The ratio of pleural fluid protein to serum protein is greater than 0.5
  • The ratio of pleural fluid LDH and serum LDH is greater than 0.6
  • Pleural LDH is greater than 200
  • Pleural fluid LDH is greater than 0.6 or ⅔ times the normal upper limit for serum. Different laboratories have different values for the upper limit of serum LDH, but examples include 200 and 300 IU/l



A transudate is a clear fluid, similar to blood without the cells. Transudates occur as a result of systemic changes, so liver failure, renal failure, heart failure, hypoglobulinaemia and ascites.


Exudates are mucky fluid, fluid with cells, or pus, or general gunk in. They are from local lung changes, so caused by trauma, lung cancer, pulmonary embolism, pneumonia.

Clinical and Associated Features