Pleural Fluid Analysis

A pleural effusion is a collection of fluid in the pleural space. Pleural effusion are the result of :

• Increased fluid accumulation
• Decreased lymphatic clearance of fluid
  • Obstruction to drainage
  • Increased venous pressure
• Pleural effusions are most commonly caused by CCF, Infection (pneumonia) and Malignancy
• Pleural Fluid may be examined by a pleural tap or thoracocentesis

Common causes

• Exudate (local disease) (High protein). Local factors influence the accumulation or clearance of fluid.
  • Malignancy – Lung, breast, pleural.
  • Infection – Pneumonia, empyema, pleuritis, viral disease
  • Autoimmune – Rheumatoid, SLE
  • Vascular – PTE
  • Cardiac – Pericarditis, CABG
  • Respiratory – Haemothorax, Chylothorax
  • Abdominal – Subphrenic abscess
• Transudate (systemic illness) (Low protein <30g). Imbalance between oncotic and hydrostatic pressures
  • Cardiac – CCF, PTE
  • Liver – Ascites, Cirrhosis
  • Renal – Glomerulonephritis, Nephrotic syndrome
  • Ovarian – Meigs syndrome
  • Autoimmune – Sarcoid
  • Thyroid – Myxoedema

Differentiation of exudate and transudate fluid

• Aims to identify local from systemic illness. Common causes can then be actively sought and treated
• Use Light’s criteria is moderately sensitive for differentiation, further tests are then required to further define the exudate

Pleural fluid from thoracocentesis

Pleural fluid laboratory findings

• Lights criteria (High protein and LDH = exudate), determines presence of exudate with protein and LDH levels
  • Pleural fluid protein to serum protein ratio >0.5
  • Pleural fluid LDH to serum LDH ratio >0.6
  • Pleural fluid level >2/3 of upper value for serum LDH
• Additional criteria – Confirm exudate if results equivocal
  • Serum albumin – pleural fluid albumin <1.2g/dL

Further tests

If exudate is confirmed, further testing required to evaluate cause of exudate

• Differential cell count (predominance of white cells)
  • Neutrophils – PTE, pancreatitis, pneumonia, empyema
  • Lymphocytes – Cancer, TB pleuritis
  • Eosinophila – Pneumothorax, haemothorax, asbestosis, Churg-Strauss
  • Mononuclear cells – Chronic inflammatory process
• Gram stain and culture and cytology
  • Use blood culture bottles and specimen jars – especially if chronic illness or suspect TB or fungus
  • Cytology useful in cases of suspected malignancy
• Glucose
  • Low
    • Common: Infection (pneumonia) and malignancy
    • Rare: TB, haemothorax, Churg-Strauss
• LDH level – This is classically high in exudates
  • Repeated testing confirms continuation or cessation of process
    • Increasing LDH (ongoing inflammation)
    • Decreasing LDH (cessation of process)
• Pleural fluid pH (Low glucose and pH = infection or malignancy)
  • Taken if suspect pneumonic or malignant process (Low glucose)
  • <7.20 with pneumonia…Drain the fluid
  • <7.20 with malignancy …Life expectancy 30 days
• Amylase
  • Useful if suspect pancreatitis as cause

References and Links

• CCC – Pleural effusion
• CCC – Pleural tap

Mike Cadogan

Associate Professor Curtin Medical School, Curtin University. Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |