What are the Lights criteria (Light's criteria) used for in differentiating pleural effusions?

Light's Criteria for Pleural Effusion Classification

Light's criteria are used to differentiate exudative from transudative pleural effusions by measuring pleural fluid and serum protein and lactate dehydrogenase (LDH) levels. 1

The Three Criteria

A pleural effusion is classified as an exudate if it meets ANY ONE of the following thresholds: 1, 2

• Pleural fluid to serum protein ratio >0.5
• Pleural fluid to serum LDH ratio >0.6
• Pleural fluid LDH >0.67 (or 2/3) of the upper limit of normal serum value

Meeting just one criterion is sufficient to classify the effusion as exudative. 1

Performance Characteristics

Light's criteria demonstrate excellent sensitivity but moderate specificity: 1, 2

• Sensitivity: 98% - highly effective at detecting exudates and avoiding missed diagnoses of serious conditions like malignancy or infection 1
• Specificity: 72% - lower specificity means approximately 25-30% of transudates (especially cardiac and hepatic) are misclassified as exudates 1, 3, 4
• Positive likelihood ratio: 3.5 2
• Negative likelihood ratio: 0.03 2

The criteria were deliberately designed to maximize exudate detection to prevent missing potentially life-threatening conditions. 1

When Serum Samples Are Unavailable

If you cannot obtain serum samples, use this alternative "or" rule with equivalent discriminative capacity: 1, 3

• Pleural fluid LDH >67% of upper limit of normal serum LDH OR
• Pleural fluid cholesterol >55 mg/dL 1

Correcting Misclassified "False Exudates"

When Light's criteria suggest an exudate but clinical suspicion strongly favors heart failure or cirrhosis (especially in patients on diuretics), apply these corrective measures: 1, 5, 4

• Albumin gradient (serum albumin minus pleural fluid albumin) >1.2 g/dL - correctly reclassifies approximately 80% of false exudates back to transudates 1, 5
• Albumin ratio (pleural fluid/serum albumin) <0.6 - alternative method with similar accuracy 1
• NT-proBNP measurement in pleural fluid (>1500 μg/mL) or serum identifies heart failure-related effusions with 92-94% sensitivity and 88-91% specificity 5

Clinical Application Algorithm

1. Obtain pleural fluid and serum samples for protein and LDH measurement 1
2. Apply Light's criteria - if any one criterion is met, classify as exudate 1, 2
3. If exudate is identified: pursue additional diagnostic workup for malignancy, infection, tuberculosis, or inflammatory conditions 2, 5
4. If transudate is identified: treat underlying heart failure or cirrhosis, typically with diuretics; further investigation usually unnecessary 1
5. If results are borderline and clinical context suggests heart failure/cirrhosis: calculate albumin gradient or measure NT-proBNP to avoid misclassification 1, 5

Critical Pitfalls to Avoid

• Diuretic therapy is the most common cause of transudate misclassification as exudate - always check albumin gradient in these patients 1, 4
• Do not rely on imaging alone (CT, ultrasound, MRI) to differentiate transudates from exudates - biochemical analysis is essential 5
• Values near cutoff thresholds require clinical correlation and consideration of albumin gradient 1
• Approximately 80% of transudates are due to heart failure, followed by liver cirrhosis (10%) 1, 5