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Pleural Fluid

Light's Criteria

March 4th, 2023


Light's criteria are used to determine the nature of a pleural fluid sample, and narrow down the differential diagnosis of a pleural effusion. 
A pleural fluid sample is most likely exudate if one or more of the criteria are met.
  • Criteria

  • Pleural fluid : serum protein ratio >0.5
  • Pleural fluid : serum LDH ratio >0.6
  • Pleural fluid LDH > â…” of upper limit of normal serum LDH
  • Causes of Positive Light's Criteria

  • Malignancy
  • Infection - parapneumonic effusion, empyema, viral, fungal, TB, ruptured abscess / cyst
  • Pulmonary embolism (90% exudative)
  • Autoimmune disease - rheumatoid pleuritis, SLE
  • Pancreatitis
  • Haemothorax
  • Chylothorax
  • Asbestosis
  • Uraemia
  • Oesophageal rupture
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