💧 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 are highly sensitive for exudates but occasionally misclassify a transudate (e.g., diuretic-treated heart failure) as exudate. (original synthesis · not guideline verbatim)
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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.
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| Pleural protein | 40 g/L |
|---|---|
| Serum protein | 70 g/L |
| Pleural LDH | 250 U/L |
| Serum LDH | 200 U/L |
| Serum LDH upper limit of normal | 245 U/L |
→DeterminationExudate
- Protein ratio > 0.5：0.57 ✓
- LDH ratio > 0.6：1.25 ✓
- Pleural LDH > 2/3 ULN：250 vs 163 ✓
Frequently asked questions
- What is 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.
- How is Light's Criteria (Pleural Effusion) calculated? What is the core formula?
- 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.
- When is Light's Criteria (Pleural Effusion) used?
- 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).
- What are the key clinical points for Light's Criteria (Pleural Effusion)?
- 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.
- What are the limits and cautions when using Light's Criteria (Pleural Effusion)?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is Light's Criteria (Pleural Effusion) calculated in practice? Can you show a worked example?
- Inputs: Pleural protein 40 g/L, Serum protein 70 g/L, Pleural LDH 250 U/L, Serum LDH 200 U/L, Serum LDH upper limit of normal 245 U/L → Result: Determination Exudate（Protein ratio > 0.5: 0.57 ✓, LDH ratio > 0.6: 1.25 ✓, Pleural LDH > 2/3 ULN: 250 vs 163 ✓）