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Pleural fluid interpretation [advanced]

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Pleural fluid aspiration is covered here!

Types of pleural effusion

  • Serous fluid (hydrothorax)
    • May be transudate or exudate
    • Causes below
  • Blood (haemothorax)
    • Exudate
    • Caused by trauma, malignancy, PE with infarct
  • Chyle (chylothorax)
    • May be transudate or exudate
    • Leakage from thoracic duct caused by lymphoma or thoracic surgical trauma
  • Pus (empyema/ pyothorax)  
    • Exudate with pH <7.2
    • Secondary to pneumonia/ abscess


  • Straw: serous effusion (clear = transudate; cloudy = exudate)
  • Blood stained: trauma, malignancy, PE with infarct
  • Frank blood: trauma, malignancy (esp. mesothelioma)
  • Pus: empyema
  • Food particles: oesophageal rupture

Transudate vs exudates


  • Cells
    • Cell count (normally <1×109/L) + differential: lymphocytosis occurs in tuberculosis and malignancy; neutrophilia in parapneumonic effusion, PE and pancreatitis; eosinophilia in drug reaction, asbestos, parasite infection, Churg-Strauss
    • MC&S: identify infective agents
    • Cytology: identify malignant causes
  • Tests for transudate vs exudates (see table above)
    • Protein
    • LDH
  • pH (normally  Ģ“ 7.6): if pH <7.2, empyema is likely and requires a chest drain to be inserted
  • Other tests to consider
    • Glucose (normally similar to serum levels): low in MEAT: Malignancy, Empyema, Arthritis (rheumatoid), TB
    • Amylase (normally similar to serum levels): raised in pancreatitis
    • Ziehl-Neelsen stain: positive in tuberculosis
    • Haematocrit: if bloody effusion haematocrit is <1%, it is insignificant
    • Triglycerides (normal <50mg/dl), cholesterol & chylomicrons: chylomicrons present and triglycerides raised (>110mg/dl) in chylothorax; cholesterol is used to differentiate a pseudocyclothorax (>200mg/dl)
    • Rheumatoid factor & complement: raised in rheumatic causes

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