💧 Light's Criteria (Pleural Effusion)
Light's criteria distinguish a pleural effusion as exudate or transudate; meeting any one of the three criteria classifies the fluid as an exudate.
Light's Criteria (Pleural Effusion)
Pleural protein (g/L)
Serum protein (g/L)
Pleural LDH (U/L)
Serum LDH (U/L)
Serum LDH upper limit of normal (U/L)
When to use
Enter pleural and serum protein, pleural and serum LDH, and the serum LDH upper limit of normal; the tool evaluates all three criteria. An exudate prompts a search for local pleural disease (infection, malignancy, inflammation).
How it works
Exudate if any: pleural/serum protein > 0.5; OR pleural/serum LDH > 0.6; OR pleural LDH > 2/3 × serum LDH upper limit of normal. Otherwise transudate.
Key points
- Light's criteria are highly sensitive for exudates but occasionally misclassify a transudate (e.g., diuretic-treated heart failure) as exudate. (original synthesis · not guideline verbatim)
- When a transudate is strongly suspected clinically yet criteria are only marginally met, a serum-pleural albumin gradient > 12 g/L supports a transudate.
- Always interpret alongside fluid appearance, cell count/differential, glucose, pH, cytology, and microbiology rather than in isolation.
References
- Light RW, et al. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507-513.
- Roberts ME, et al. BTS Guideline for pleural disease 2023. Thorax. 2023;78(Suppl 3):s1-s42.
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.