Light’s Criteria Calculator — Exudate vs Transudate Pleural Effusion
Classify a pleural effusion as exudate or transudate with Light’s criteria from pleural and serum protein, pleural and serum LDH and the lab upper limit of normal for serum LDH.
How it works
Exudate if ANY of: pleural/serum protein > 0.5 · pleural/serum LDH > 0.6 · pleural LDH > ⅔ of the upper limit of normal for serum LDH. Transudate if none is met.
Light’s criteria (Light et al., Annals of Internal Medicine 1972) remain the standard first step in pleural fluid analysis. Transudates come from imbalanced hydrostatic or oncotic pressure — mainly heart failure, cirrhosis and nephrotic syndrome — while exudates come from inflammation, infection, malignancy or other local disease and usually need further investigation. The criteria identify almost all exudates (about 98% sensitivity) but label about a quarter of transudates as exudates, especially in patients with heart failure on diuretics. When an apparent exudate is suspected to be a transudate, a serum–pleural albumin gradient above 1.2 g/dL (or protein gradient above 3.1 g/dL) favors a transudate.
Worked example
Pleural protein 2.5 g/dL with serum protein 6.5 g/dL gives a ratio of 0.38 (not met). Pleural LDH 160 U/L with serum LDH 300 U/L gives 0.53 (not met). Two-thirds of the lab upper limit of 222 U/L is 148 U/L, and pleural LDH 160 exceeds it (met). With 1 of 3 criteria met, the effusion is an Exudate.
Sources: Light RW et al. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med 1972;77:507–513 · Roberts ME et al. British Thoracic Society Guideline for pleural disease. Thorax 2023
Frequently asked questions
How many of Light’s criteria are needed for an exudate?
Only one. An effusion that meets any one of the three criteria is classified as an exudate; a transudate meets none.
What causes a transudate versus an exudate?
Transudates are typically due to heart failure, cirrhosis (hepatic hydrothorax), nephrotic syndrome or hypoalbuminemia. Exudates are caused by pneumonia (parapneumonic effusion), cancer, tuberculosis, pulmonary embolism, autoimmune disease and other local processes.
Why might a heart failure effusion look like an exudate?
Diuretics concentrate the pleural fluid protein and LDH, so about 25% of transudates meet a criterion. A serum–pleural albumin gradient over 1.2 g/dL or protein gradient over 3.1 g/dL supports a transudate in that case.
What upper limit of normal should I use for LDH?
Use your own laboratory’s reference upper limit for serum LDH, because it varies between assays. The calculator compares the pleural LDH with two-thirds of that value.
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Clinical decision tool for education and for use by trained clinicians alongside clinical judgement — not a diagnosis and not a substitute for medical assessment. Scores and cut-offs come from the cited validation studies and may not apply to every patient; follow local protocols.