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 fluidTest indicatedInterpretation

Comparison to serum Haematocrit

  • <1% - non-significant
  • 1-20% - Cancer, PTE, trauma, pneumonia
  • >50% - Haemothorax
Cloudy or turbidTriglycerides>110mg/dL-chylothorax
Putrid odourMCSPossible anaerobic infection

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 700 6

Critical Care


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 |

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