Pleural effusion

Pleural effusions can be divided into transudates and exudates. Light’s criteria is used to distinguish between them. 

Light’s criteria:
Pleural fluid is an exudate if one or more of the following criteria are met:
  • Pleural fluid protein divided by serum protein is >0.5 
  • Pleural fluid LDH divided by serum LDH is >0.6 
  • Pleural fluid LDH is >2/3 the upper limits of laboratory normal value for LDH 

Diagnostic pleural taps should ideally be done under US guidance and the fluid sent for: 
  • All cases:
    • Protein
    • LDH
    • Gram stain
    • Cytology and cell count 
    • Microbiological culture
  • Specific cases:
    • pH – if pleural infection suspected 
    • Amylase – only if oesophageal rupture or pancreatic-associated effusion suspected 
    • Glucose – useful in diagnosis of rheumatoid effusion 
    • Triglycerides and cholesterol – if effusion milky and chylothorax suspected
    • Haematocrit – if haemothorax suspected

Causes of exudative pleural effusions: 
  • Common 
    • Malignancy 
    • Parapneumonic 
    • TB 
  • Less common 
    • PE 
    • RA 
    • Benign asbestos effusion 
    • Pancreatitis
    • Post MI
    • Post CABG 
  • Rare 
    • Yellow nail syndrome 
    • Drugs
      • Methotrexate
      • Amiodarone 
      • Phenytoin 
      • Nitrofurantoin 
      • Beta blockers 
      • Penicillamine
      • Cyclophosphamide
    • Fungal infections 

Causes of transudative pleural effusions 

  • pH 
    • pH <7.2 indicates the need for drainage 
    • pH <7.3 
      • malignant effusions 
      • pleural infection
      • connective tissue diseases 
      • TB 
      • Oesophageal rupture 

  • Glucose
    • Low (<3.4mmol/l) 
      • Parapneumonic 
      • RA 
      • TB 
      • Malignancy 
      • Oesophageal rupture 
    • Very low (<1.6mmol/l) 
      • RA 
      • Empyema 

When PE's cause effusions,  the dyspneoa is often disproportionate to the size of the effusion. 
Post CABG effusions: left >right 

  • If clinically a transudate is suspected, treat cause
  • If cause unclear, perform a diagnostic tap and then treat cause as appropriate 
  • If pleural aspiration (thoracocentesis) is indicated:
    • Stop when patient develops chest discomfort or cough or when 1.5l has been drained
  • Malignant pleural effusion (symptomatic) 
    • Unless very short life expectancy, chest drain as high probability of recurrence if theraupetic aspiration alone 
    •   Consider pleurodesis once effusion drained and lung re-expanded
  • Drain large effusions in a controlled fashion to decrease risk of re-expansion pulmonary oedema

Small print gem: there needs to be around 200mls of fluid for it to be detectable on a PA CXR. 

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