Respiratory
Pleural Fluid: Exudate or Transudate (Light's Criteria) Calculator
Classifies a pleural effusion as an exudate or a transudate from pleural and serum protein and LDH.
Formula
Exudate if any one of:
Pleural fluid protein ÷ serum protein over 0.5
Pleural fluid LDH ÷ serum LDH over 0.6
Pleural fluid LDH over two-thirds of the upper limit of normal for serum LDH
Transudate if none is met
How it is used
- Highly sensitive for an exudate (98%) but less specific (69–71%) across over 7,000 patients; heart failure made up 87–92% of the transudates that both Light’s criteria and the authors’ blood-free combination wrongly called exudates (Porcel 2026).
- When heart failure is likely, check the serum minus pleural albumin gradient: over 1.2 g/dL points to a transudate; a gradient of 1.2 or less identified exudates with 72% sensitivity and 90% specificity (Porcel 2026).
- The LDH limit depends on the laboratory’s reference range, so enter your own upper limit of normal (Addala 2025).
Key points
- Draw serum protein and LDH at the same time as the thoracentesis: the ratios need simultaneous samples.
Related one-pagers
- Chest Tubes and PneumothoraxLight's criteria: protein ratio >0.5, LDH ratio >0.6 or LDH >two-thirds of the upper normal serum value means exudate; pH ≤7.2 in suspected infection means drain.
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For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.