Light’s Criteria Calculator (Exudate vs Transudate)
Light’s criteria separate an exudative from a transudative pleural effusion using paired pleural-fluid and serum values. The effusion is an exudate if any one of three conditions is met: a protein ratio over 0.5, an LDH ratio over 0.6, or a pleural LDH above two-thirds of the upper limit of normal serum LDH.
Quick answer
Light’s criteria classify a pleural effusion as an exudate if any one of three conditions is met: a pleural-to-serum protein ratio over 0.5, a pleural-to-serum LDH ratio over 0.6, or a pleural LDH above two-thirds of the upper limit of normal serum LDH. If none are met, the effusion is a transudate.
Protein
LDH
How to use the Light’s Criteria calculator
- 1Enter the protein values. Enter the pleural-fluid and serum total protein (g/dL).
- 2Enter the LDH values. Enter the pleural-fluid LDH, serum LDH, and the upper limit of normal for serum LDH (all IU/L).
- 3Read exudate vs transudate. The tool checks all three criteria and reports exudate if any one is met, or transudate if none are — with the individual ratios shown for transparency.
Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.
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Book a consultationFrequently asked questions
When is a pleural effusion an exudate by Light’s criteria?⌄
An effusion is an exudate if any one of the three criteria is met: protein ratio over 0.5, LDH ratio over 0.6, or pleural LDH above two-thirds of the serum LDH upper limit. Meeting even one is enough.
Why do Light’s criteria sometimes misclassify a transudate as an exudate?⌄
The criteria are very sensitive for exudates but can mislabel a transudate (for example in a patient on diuretics for heart failure) as an exudate. In borderline cases the serum-to-pleural albumin gradient is sometimes used to help. A clinician interprets the result.
Why does the distinction matter?⌄
Transudates usually reflect systemic causes such as heart failure or cirrhosis, while exudates point to local processes such as infection, malignancy or inflammation — so the classification guides which further tests are needed.
Is my data stored?⌄
No. The calculator runs entirely in your browser; nothing you enter is sent to a server.
Sources & validation
This calculator reproduces the published Light’s Criteria score, validated for classifying a pleural effusion as an exudate or a transudate.
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