Pleural fluid interpretation [advanced]

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

Appearances

• 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

Tests

• 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  ̴ 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