Light's criteria (pleural effusion)
Classify a pleural effusion as exudate or transudate
Sources last checked: 2026-10-13
Light's criteria (pleural effusion)
Classify a pleural effusion as exudate or transudate
Highly sensitive for exudates but labels roughly one in four transudates as exudates, particularly in heart failure after diuretics. A serum minus pleural albumin gradient > 12 g/L (1.2 g/dL) then favours a transudate.
Interpretation
| Transudate | No criterion met. Transudates are typically caused by systemic conditions (heart failure, cirrhosis, nephrotic syndrome, hypoalbuminaemia). |
|---|---|
| Exudate | At least one criterion met. Exudates typically have local pleural causes (parapneumonic, malignant, tuberculous, PE-related, inflammatory). In heart failure on diuretics the criteria can misclassify a transudate as an exudate; the serum–pleural albumin gradient helps separate the two (see caveat). |
What is the Light's criteria (pleural effusion)?
Light's criteria classify a pleural effusion as an exudate if any of three conditions is met: pleural/serum protein ratio > 0.5, pleural/serum LDH ratio > 0.6, or pleural LDH above two-thirds of the upper normal serum LDH. Light and colleagues described them in 1972, and five decades later they remain the reference standard for separating exudates from transudates.
When to use it
- Every diagnostic pleural aspiration where the cause is not obvious
- Deciding whether an effusion needs further local work-up (cytology, microbiology, pleural biopsy)
- Teaching the first step in pleural fluid interpretation
When not to use it
- When serum protein and LDH were not taken close to the time of aspiration
- Frank empyema or obvious haemothorax, where the classification adds little
- As the only test in suspected heart-failure effusion after diuretics — the albumin gradient is more informative there
How it is calculated
Exudate if ANY of: • pleural/serum protein ratio > 0.5 • pleural/serum LDH ratio > 0.6 • pleural LDH > 2/3 of the upper limit of normal for serum LDH Transudate if none is met.
Clinical pearls and limitations
- Highly sensitive for exudates but labels roughly one in four transudates as exudates, particularly in heart failure after diuretics. A serum minus pleural albumin gradient > 12 g/L (1.2 g/dL) then favours a transudate.
- The criteria are deliberately sensitive (≈ 98 % for exudates) at the cost of misclassifying about a quarter of transudates.
- Diuretics concentrate pleural protein and LDH, so heart-failure effusions often look like exudates.
- A serum minus pleural albumin gradient > 12 g/L (1.2 g/dL) favours a transudate in that situation.
- Protein can be entered in g/L or g/dL as long as pleural and serum use the same unit, because only the ratio matters.
Frequently asked questions
What are Light's criteria?
Exudate if pleural/serum protein > 0.5, pleural/serum LDH > 0.6, or pleural LDH > 2/3 of the upper limit of normal for serum LDH. Meeting none means transudate.
Why do Light's criteria misclassify transudates?
They were tuned to miss as few exudates as possible. Diuretic treatment in heart failure in particular raises pleural protein and LDH into the exudative range.
Is there a test without serum samples?
A pleural-fluid-only combination (protein > 30 g/L, LDH above two-thirds of the ULN, or cholesterol > 55 mg/dL) performed similarly to Light's criteria in a large Spanish cohort.
Which upper limit of normal for LDH should I use?
Your own laboratory's upper reference limit for serum LDH, because LDH assays and ranges differ between laboratories.
Evidence and guidelines
- Primary source: Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med 1972;77(4):507-513.
- Porcel JM, Porcel L, Palma R, Bielsa S. Revisiting Light's criteria: a validated blood-free triple combination matches diagnostic accuracy in over 7000 patients. ERJ Open Res 2026;12(2):00821-2025.
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