Light’s Criteria (Pleural Exudate vs Transudate)
Apply Light's criteria (Light 1972) to classify a pleural effusion as exudate or transudate from paired pleural and serum protein and LDH values plus the lab's upper limit of normal for serum LDH.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Pleural fluid total protein
- 4.5 g/dL
- Serum total protein
- 6.0 g/dL
- Pleural fluid LDH
- 300 IU/L
- Serum LDH
- 200 IU/L
- Upper limit of normal for serum LDH
- 250 IU/L
Result: Exudate — meets Light's criteria (protein ratio 0.75 > 0.5; LDH ratio 1.5 > 0.6; pleural LDH 300 > 166.7 (2/3 ULN)).
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Exudate
- Any of: pleural/serum protein > 0.5; pleural/serum LDH > 0.6; pleural LDH > two-thirds the ULN of serum LDH.
- Transudate
- None of the three criteria met. Highly sensitive for exudates (can misclassify ~15-20% of transudates). A classification, not a workup order.
What you enter
- Pleural fluid total protein
- Serum total protein
- Pleural fluid LDH
- Serum LDH
- Upper limit of normal for serum LDH
What this is
Sophie Well computes the protein ratio, the LDH ratio, and the absolute pleural LDH versus two-thirds of the serum upper limit, and flags an exudate when any single criterion is met.
When to use it
Use this at the bedside after a diagnostic thoracentesis to direct the workup: transudates point at heart failure, cirrhosis, or nephrotic syndrome; exudates open the differential to infection, malignancy, pulmonary embolism, and connective-tissue disease. Light's criteria are highly sensitive for exudate but only moderately specific - heart-failure patients on diuretics can be misclassified, so pair an unexpected exudate with serum-to-pleural albumin gradient or NT-proBNP before committing to a workup.
How this is calculated
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. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.