Light’s Criteria Calculator (Pleural Effusion)
Light's Criteria Calculator (Pleural Effusion)
Classify a pleural effusion as exudate or transudate from paired pleural and serum protein and LDH — meeting any one criterion is enough to call it an exudate.
Light's Criteria (Pleural Effusion)
Protein + LDH → exudate/transudatePleural protein 3.8 g/dL, serum protein 6.8 g/dL, pleural LDH 320 U/L, serum LDH 210 U/L, LDH ULN 225 U/L
Light’s criteria
pleural protein ÷ serum protein > 0.5
pleural LDH ÷ serum LDH > 0.6
pleural LDH > ⅔ × serum LDH upper limit of normal
- protein ratio
- pleural fluid protein ÷ serum protein, paired samples on the same day
- LDH ratio
- pleural fluid LDH ÷ serum LDH
- LDH ULN
- the local laboratory’s upper limit of normal for serum LDH, not the patient’s own serum LDH
- ANY
- meeting one criterion is sufficient — the three are not averaged or combined
Worked example
Pleural protein 3.8 g/dL, serum protein 6.8 g/dL, pleural LDH 320 U/L, serum LDH 210 U/L, LDH ULN 225 U/L
Protein ratio = 3.8 ÷ 6.8 = 0.56, above 0.5 → exudate on the protein criterion alone
LDH ratio = 320 ÷ 210 = 1.52, also above 0.6 — this sample meets every Light's criterion, not just one
Light’s criteria at a glance
| Criterion | Threshold | This example |
|---|---|---|
| Protein ratio | > 0.5 | 0.56 — met |
| LDH ratio | > 0.6 | 1.52 — met |
| Absolute LDH | > ⅔ × ULN (150 U/L here) | 320 U/L — met |
Applying Light’s criteria, and where they mislead
Light’s criteria classify a pleural effusion as an exudate if pleural fluid protein exceeds half of serum protein, pleural fluid LDH exceeds 60% of serum LDH, or pleural fluid LDH exceeds two-thirds of the laboratory’s upper limit of normal for serum LDH. Any one criterion met is sufficient — there is no requirement to satisfy all three, and no averaging between them.
The criteria are about 98% sensitive for exudates, which is their real strength: a true exudate is very rarely missed. Their weakness is specificity. Roughly a quarter of transudates are misclassified as exudates, and this happens predictably in patients diuresed for heart failure — diuresis concentrates pleural fluid protein and LDH even though the underlying process remains a transudative one. In that specific situation, a serum-to-pleural-fluid albumin gradient above 1.2 g/dL, or a protein gradient above 3.1 g/dL, correctly identifies the fluid as a transudate despite a positive Light’s result.
The two categories point to different causes and different next steps. Transudates arise from altered hydrostatic or oncotic pressure — heart failure, cirrhosis, nephrotic syndrome — and are managed by treating that underlying condition rather than by investigating the pleura. Exudates arise from pleural or capillary disease — infection, malignancy, pulmonary embolism, connective tissue disease — and need further pleural fluid analysis and often imaging to find the cause.
Frequently asked questions
What are Light’s criteria?
A set of three ratios comparing pleural fluid protein and LDH against serum values. Meeting any one criterion classifies the effusion as an exudate; meeting none of them classifies it as a transudate.
How accurate are Light’s criteria?
About 98% sensitive for exudates, so true exudates are rarely missed. Specificity is weaker — roughly 25% of transudates are misclassified as exudates, most often in patients on diuretics for heart failure.
What if a heart failure patient on diuretics looks like an exudate?
Check the serum-to-pleural-fluid gradients. An albumin gradient above 1.2 g/dL, or a protein gradient above 3.1 g/dL, correctly identifies a true transudate that Light’s criteria have misclassified.
What causes a transudate versus an exudate?
Transudates come from altered hydrostatic or oncotic pressure — heart failure, cirrhosis, nephrotic syndrome — and resolve by treating that cause. Exudates come from pleural or capillary disease — infection, malignancy, pulmonary embolism, connective tissue disease — and need direct investigation.
Related calculators
References
- Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507–13.
- Porcel JM. Identifying transudates misclassified by Light’s criteria. Curr Opin Pulm Med. 2013;19(4):362–7.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.