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.