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Pleural fluid analysis (Light's criteria)

Body Fluids · also called Pleural tap

What it measures

Analyses fluid from thoracentesis to classify an effusion as transudate or exudate and find its cause. Light's criteria compare pleural and serum protein and LDH.

Specimen and preparation

Specimen: Pleural fluid in sterile, EDTA and heparinised tubes/blood-culture bottles; paired serum
Tube: Other top
Preparation: Collect paired serum protein and LDH; send pH anaerobically in a blood-gas syringe.

Reference range

Group	Range
Exudate criterion 1 (any one)	Pleural/serum protein ratio > 0.5
Exudate criterion 2	Pleural/serum LDH ratio > 0.6
Exudate criterion 3	Pleural LDH > 2/3 of serum upper reference limit
Transudate	None of the three criteria met

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Exudate: pneumonia/empyema, tuberculosis, malignancy, pulmonary embolism
High ADA (> 40 U/L) with lymphocytes: tuberculosis
pH < 7.2 or glucose < 60 mg/dL: complicated effusion, needs drainage

Causes of low values

Transudate: heart failure, cirrhosis, nephrotic syndrome

Method

Protein, LDH, glucose, pH, cell count and differential, Gram stain, culture, ADA, cytology

Good to know

Light's criteria can misclassify diuretic-treated heart failure as exudate; a serum-pleural albumin gradient > 1.2 g/dL favours transudate.

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