Light's Criteria Calculator - Pleural Effusion Classification
Enter the pleural fluid and serum protein and LDH values from thoracentesis to instantly classify the effusion as exudative or transudative using Light's criteria. Each of the three ratios is calculated and shown individually so you can see which criterion is positive. A result panel explains what the classification means and lists the most common underlying causes.
Formula
Worked example
Pleural protein 4.5 g/dL, serum protein 7.0 g/dL, pleural LDH 280 U/L, serum LDH 220 U/L, upper-normal LDH 240 U/L. Criterion 1: 4.5/7.0 = 0.643 >= 0.5 (positive). Criterion 2: 280/220 = 1.273 >= 0.6 (positive). Criterion 3: 280/240 = 1.167 >= 0.667 (positive). All three positive: exudative effusion.
What are Light's criteria?
Light's criteria are a set of three laboratory thresholds used to classify a pleural effusion as either an exudate (caused by local pleural or lung pathology) or a transudate (caused by a systemic imbalance in hydrostatic or oncotic pressure). They were described by Richard W. Light and colleagues in 1972 and remain the standard first-line test for effusion classification worldwide. The criteria require a thoracentesis (pleural fluid aspiration) with simultaneous blood sampling so that paired protein and LDH values can be compared. An effusion is classified as an exudate if it meets any one of the three criteria; all three must be negative for a transudate diagnosis. The test has sensitivity above 97% for exudates, making it the most sensitive tool available, though its specificity for exudates is lower (around 74-83%), which means some true transudates are misclassified.
How to interpret the result
An exudative result means the fluid has high protein and/or LDH relative to serum, pointing to a local process that is increasing vascular permeability or causing cell breakdown in the pleural space. The differential diagnosis is broad: pneumonia (parapneumonic effusion or empyema), malignancy, tuberculosis, pulmonary embolism, rheumatoid arthritis, lupus, viral pleuritis, pancreatitis, and esophageal rupture are the most common causes. Further workup typically includes pleural fluid pH, glucose, cytology, culture, cell differential, and chest imaging. A transudative result suggests the pleural membranes are intact and normal, with fluid accumulating because of elevated hydrostatic pressure or low plasma oncotic pressure: congestive heart failure accounts for roughly 40% of all transudative effusions, followed by hepatic hydrothorax (cirrhosis) and nephrotic syndrome. Hypoalbuminemia from any cause, hypothyroidism, and peritoneal dialysis can also produce transudates. Note: patients on long-term diuretic therapy for heart failure can develop pseudo-exudates where the serum proteins are concentrated by fluid loss, making a true transudate appear exudative by Light's criteria. The serum-to-effusion albumin gradient (SEAG > 1.2 g/dL) or the serum-to-effusion protein gradient (> 3.1 g/dL) can help unmask these cases.
Common causes by effusion type
Exudative effusions arise from conditions that inflame the pleura or block lymphatic drainage. The most frequent causes in adults are bacterial pneumonia (parapneumonic), malignancy (especially lung cancer, breast cancer, and lymphoma), pulmonary embolism, and tuberculosis. Less common but important causes include autoimmune diseases (rheumatoid arthritis, systemic lupus), pancreatitis, hemothorax, chylothorax, drug reactions, post-cardiac injury syndrome, and benign asbestos pleural effusion. Transudative effusions result from systemic hemodynamic disturbances. Congestive heart failure is the single most common cause of bilateral transudative effusions. Hepatic cirrhosis with ascites, nephrotic syndrome, hypoalbuminemia from malnutrition, hypothyroidism, atelectasis (reduced pleural pressure), peritoneal dialysis, constrictive pericarditis, and urinothorax (urine leaking into the pleural space) complete the main list.
Sensitivity, specificity, and alternatives to Light's criteria
Light's criteria perform very well for detecting exudates (sensitivity approximately 97-100%) but are only moderately specific (approximately 74-83%), meaning about 1 in 4 true transudates may be mislabeled as exudates. This matters most in heart failure patients treated with diuretics. Several supplementary tests have been proposed. The serum-to-effusion albumin gradient (SEAG) greater than 1.2 g/dL, or the serum-to-effusion protein gradient greater than 3.1 g/dL, reclassifies many apparent exudates (by Light's criteria) back to transudates in diuretic-treated heart failure. Pleural fluid cholesterol above 60 mg/dL and pleural fluid LDH above 200 U/L (as combined criteria proposed by Romero-Candeira) also have high accuracy. Despite these alternatives, Light's criteria remain the internationally accepted starting point because they are simple, reproducible, and have the highest sensitivity for detecting exudates - missing an exudate (e.g., an early empyema or malignant effusion) carries greater clinical risk than over-investigating a suspected exudate.
Light's criteria thresholds and interpretation
| Criterion | Threshold | Positive if... | Significance |
|---|---|---|---|
| Pleural protein / Serum protein | >= 0.5 | Ratio >= 0.5 | Protein-rich fluid suggests pleural inflammation or increased vascular permeability |
| Pleural LDH / Serum LDH | >= 0.6 | Ratio >= 0.6 | Elevated LDH in fluid reflects local cellular damage or active inflammation |
| Pleural LDH vs upper-normal serum LDH | >= 2/3 (0.667) | Fraction >= 0.667 | Absolute pleural LDH exceeding 2/3 of normal serum range confirms local process |
An effusion is classified as an exudate if ANY ONE criterion is met. All three must be negative for a transudate.
Frequently asked questions
What happens if only one of Light's criteria is positive?
The effusion is classified as an exudate. Light's criteria use an OR logic: any single positive criterion is sufficient. One positive criterion out of three is the most common pattern in early or mild exudative effusions, such as a small parapneumonic effusion or early malignant effusion.
Can Light's criteria misclassify heart failure effusions as exudates?
Yes. Patients with congestive heart failure who have been on diuretics can concentrate their pleural proteins and LDH to levels that satisfy one of Light's criteria even though the effusion is truly a transudate. This affects up to 30% of heart failure thoracenteses after diuresis. The serum-to-effusion albumin gradient (SEAG > 1.2 g/dL) or the serum-to-effusion protein gradient (> 3.1 g/dL) can help correct the classification in these cases.
Why must blood and pleural fluid be sampled at the same time?
Light's criteria are all ratios: pleural value divided by the paired serum value. If the blood sample is taken hours later, serum protein or LDH may have changed, distorting the ratios. Contemporary guidelines recommend drawing blood within a few hours of the thoracentesis, ideally from the same sitting.
What additional tests should follow a positive (exudate) result?
After classifying an effusion as exudative, further pleural fluid analysis typically includes: pH (low pH < 7.2 suggests empyema or complicated parapneumonic effusion), glucose (low glucose < 60 mg/dL is seen in empyema, malignancy, and rheumatoid effusions), cell count and differential (neutrophils predominate in bacterial infection; lymphocytes in tuberculosis and malignancy; eosinophils suggest air, blood, parasites, or asbestosis), cytology (for malignant cells), Gram stain and culture, and triglycerides if chylothorax is suspected. Chest CT and bronchoscopy may follow based on the clinical picture.
What is the upper-normal LDH value and where do I find it?
The upper limit of normal (ULN) for serum LDH varies by laboratory depending on the assay method and reagents used. A common adult reference range is 100-240 U/L, giving a ULN of 240 U/L, but your laboratory's reference range should always take priority. This value is printed on the laboratory report alongside the LDH result. If in doubt, call your lab or check the reference interval listed on the report.
How accurate are Light's criteria overall?
For identifying exudates, Light's criteria have a sensitivity above 97% - they very rarely miss a true exudate. Specificity for exudates is lower, around 74-83%, meaning some true transudates are misclassified as exudates (false exudates). The overall accuracy in the original 1972 study and in subsequent meta-analyses is approximately 93-96%, making this one of the most validated tests in respiratory medicine.