Light's Criteria

Classify a pleural effusion as exudate or transudate

Pleural fluid protein (g/dL)

Serum protein (g/dL)

Pleural fluid LDH (U/L)

Serum LDH (U/L)

Serum LDH upper limit of normal (U/L)

Enter all five values to classify

Evidence

Original publication

Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. An effusion is an exudate if the pleural/serum protein ratio exceeds 0.5, the pleural/serum LDH ratio exceeds 0.6, or pleural LDH exceeds two-thirds of the upper limit of normal for serum — sensitivity for exudates was ~99%.

Annals of Internal Medicine, 1972

Open source

Guideline

BTS and ATS pleural-disease guidance endorse Light’s criteria as the first-line test; because they misclassify ~25% of transudates as exudates (often in diuretic-treated heart failure), a serum–pleural fluid protein or albumin gradient is recommended when the clinical picture suggests a transudate.

Last reviewed: 2026-09

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