Pleural fluid analysis
Calculate all three Light’s criteria to classify pleural fluid as an exudative or transudative pattern, with optional serum–pleural gradients for possible pseudoexudates.
Educational clinical decision support. Light’s criteria classify fluid as an exudative or transudative pattern; they do not diagnose the cause, determine whether thoracentesis is safe, or replace pleural-fluid pH and urgent clinical assessment when infection is suspected.
Paired serum + pleural-fluid analysis
Use serum and pleural samples obtained at the same clinical time whenever possible. Enter the laboratory-specific serum LDH upper limit—not the measured serum LDH—in the ULN field.
Required values
Optional pseudoexudate context
Enter both albumin values if a transudative cause—especially heart failure after diuresis or hepatic hydrothorax—is clinically suspected despite an exudative Light’s classification.
The rule
The criteria are intentionally sensitive. A single positive result classifies the effusion as exudative, but the result must be interpreted alongside the pretest clinical picture and subsequent diagnostic workup.
Pleural protein ÷ serum protein > 0.50
Pleural LDH ÷ serum LDH > 0.60
Pleural LDH > two-thirds of the serum LDH laboratory ULN
| Pattern | Common mechanisms | Representative causes |
|---|---|---|
| Transudative | Altered systemic hydrostatic or oncotic pressure | Heart failure, cirrhosis, hypoalbuminemia, nephrotic syndrome |
| Exudative | Local pleural inflammation, vascular permeability, or impaired lymphatic drainage | Malignancy, pleural infection, pulmonary embolism, autoimmune pleuritis |
Important limitation
Diuresis can concentrate pleural protein and LDH, causing an effusion from heart failure to cross an exudative threshold. When the overall clinical picture strongly favors a transudative cause, gradients may help—but they should not erase an exudative result or exclude a second process.
Preferred when available
Serum − pleural albumin >1.2 g/dL
Secondary option
Serum − pleural protein >3.1 g/dL
The classification does not identify malignancy, infection, tuberculosis, chylothorax, hemothorax, or pulmonary embolism. Those questions require targeted fluid studies, imaging, and clinical reasoning.
When parapneumonic effusion or empyema is suspected, frank pus, pleural-fluid pH, glucose, LDH, microbiology, imaging, and the patient’s clinical condition guide drainage decisions. Light’s classification is not a substitute.
Frequently asked questions
Pleural fluid is classified as exudative when any one of the following is present: pleural-to-serum protein ratio greater than 0.5, pleural-to-serum LDH ratio greater than 0.6, or pleural LDH greater than two-thirds of the laboratory upper limit of normal for serum LDH.
Yes. Light’s framework requires only one positive criterion. The number of positive criteria does not identify the cause or measure disease severity.
This term commonly describes a clinically transudative effusion—often related to heart failure after diuresis—that meets Light’s exudative criteria because protein and LDH have become concentrated. A coexisting exudative process must still be considered.
When a transudate is strongly suspected but Light’s criteria indicate an exudate, a serum-to-pleural albumin gradient greater than 1.2 g/dL may support a misclassified transudate. If albumin is unavailable, a serum-to-pleural protein gradient greater than 3.1 g/dL can provide secondary context.
No. It separates exudative from transudative patterns. Diagnosis requires the clinical presentation, imaging, cell differential, pH, glucose, microbiology, cytology, and targeted investigations.
Paired samples obtained at the same clinical time are preferred because both ratios compare pleural-fluid values with serum values.
Evidence base
Evidence reviewed August 16, 2026. Laboratory interpretation should follow local standards and current institutional pleural-disease pathways.
PulmTools pulmonology cluster
Move between screening exposure, nodule follow-up, malignancy-risk estimation, and pleural-fluid classification without leaving the Pulmonology suite.