What are Light's criteria for differentiating between transudative and exudative pleural effusions?

Light's Criteria for Pleural Effusion Classification

Light's criteria classify a pleural effusion as exudative if at least one of three parameters is met: pleural fluid to serum protein ratio >0.5, pleural fluid to serum LDH ratio >0.6, or pleural fluid LDH >0.67 (two-thirds) of the upper limit of normal serum LDH. 1, 2

The Three Criteria

A pleural effusion is classified as an exudate when it meets at least one of the following:

• Pleural fluid/serum protein ratio >0.5 1, 2
• Pleural fluid/serum LDH ratio >0.6 1, 2
• Pleural fluid LDH >0.67 (or two-thirds) of the upper limit of normal serum LDH 1, 2

If none of these criteria are met, the effusion is classified as a transudate. 1

Performance Characteristics

Light's criteria demonstrate excellent sensitivity but more limited specificity:

• Sensitivity: 98% for detecting exudative effusions 1, 2
• Specificity: 72% for identifying exudative effusions 1, 2
• Positive likelihood ratio: 3.5 1, 2
• Negative likelihood ratio: 0.03 1, 2

The high sensitivity was intentionally designed to maximize detection of exudates, avoiding missed diagnoses of serious conditions like malignancy or infection. 1

Alternative Approach When Serum Unavailable

If serum samples cannot be obtained, use these alternative thresholds:

• Pleural fluid LDH >67% of the upper limit of normal serum LDH, OR 3
• Pleural fluid cholesterol >55 mg/dL 3

Common Pitfalls and How to Address Them

Misclassification of Transudates as Exudates

The most significant limitation is that 25-30% of cardiac and hepatic transudates are incorrectly classified as exudates, particularly in patients receiving diuretics. 3, 4

To correct false exudates, use:

• Serum-pleural fluid albumin gradient >1.2 g/dL (correctly reclassifies approximately 80% of false exudates) 2
• Pleural fluid/serum albumin ratio <0.6 2
• NT-proBNP >1500 μg/mL in pleural fluid or serum (sensitivity 92-94%, specificity 88-91% for heart failure) 2

Analytical Platform Variability

Different laboratory analyzers can produce discordant results in 18% of cases, particularly affecting LDH measurements in pleural fluid versus serum. 5 When results are borderline or clinically inconsistent, consider using the serum-effusion albumin gradient as confirmation. 4

Clinical Application Algorithm

1. Obtain paired pleural fluid and serum samples for protein and LDH measurement 1, 2
2. Calculate all three Light's criteria ratios 1, 2
3. If any one criterion is positive → classify as exudate and pursue extensive workup for malignancy, infection, or inflammatory conditions 1, 2
4. If all criteria are negative → classify as transudate and direct therapy toward heart failure, cirrhosis, or nephrosis 2, 6
5. If exudate classification seems inconsistent with clinical picture (especially in patients on diuretics with suspected heart failure), calculate serum-pleural fluid albumin gradient or measure NT-proBNP 2, 3