What are Light's criteria for distinguishing exudative from transudative pleural effusions?

Light's Criteria for Pleural Effusion Classification

Light's criteria classify a pleural effusion as an exudate if any one of three thresholds is met: pleural fluid-to-serum protein ratio >0.5, pleural fluid-to-serum LDH ratio >0.6, or pleural fluid LDH >0.67 times the upper limit of normal serum LDH. 1

Definition and Application

• Simultaneous collection of both pleural fluid and serum samples is required to measure protein and LDH levels for proper application of the criteria 1
• Meeting just one of the three thresholds is sufficient to label the effusion exudative; meeting none indicates a transudate 1
• The criteria were specifically designed to maximize detection of exudates to avoid missing potentially serious conditions like malignancy or infection 2

Performance Characteristics

• Light's criteria demonstrate 98% sensitivity for identifying exudative effusions 1, 3
• Specificity is 70-80% for identifying exudates, which is notably lower than the sensitivity 1, 3
• The positive likelihood ratio is 3.5 and the negative likelihood ratio is 0.03 1
• Approximately 25-30% of cardiac or hepatic transudative effusions are misclassified as exudates, particularly in patients receiving diuretics 1, 3

Alternative Approach When Serum Is Unavailable

• If venipuncture is not possible, an "or" rule using pleural fluid LDH >67% of the upper limit of normal serum LDH and pleural fluid cholesterol >55 mg/dL provides discriminative capacity comparable to Light's criteria 1
• The abbreviated Light criteria (omitting the pleural fluid-to-serum LDH ratio) yields 95.4% sensitivity and 83.3% specificity when serum LDH is unavailable 4

Correcting Misclassification: The Pseudoexudate Problem

When Light's criteria suggest an exudate but clinical suspicion strongly points to a transudate (especially heart failure or cirrhosis), use the following reclassification tools:

• Serum-effusion albumin gradient (SEAG): serum albumin minus pleural fluid albumin >1.2 g/dL reclassifies approximately 80% of false-positive exudates as transudates with 97.5% accuracy 1, 5
• Albumin ratio: pleural fluid albumin divided by serum albumin <0.6 indicates a transudate 1
• NT-proBNP: pleural fluid or serum NT-proBNP >1500 pg/mL confirms heart failure-related effusion with 92-94% sensitivity and 88-91% specificity 1
• The combination of SEAG >1.2 g/dL and serum-pleural effusion protein gradient >3.1 g/dL together yielded 100% sensitivity in heart failure and 99% sensitivity in hepatic hydrothorax when identifying pseudoexudates 6

Critical Pitfalls and Caveats

• Diuretic therapy is the most common cause of misclassification, concentrating pleural fluid proteins and making transudates appear exudative 1, 6
• Analytical platform variability can cause up to 18% discordance in classification between different laboratory analyzers, particularly affecting LDH measurements 1, 7
• The misclassification rate is approximately 25%, primarily due to transudates being incorrectly labeled as exudates 3
• Certain conditions such as non-expansile lung, chylothorax, and superior vena cava syndrome may produce either transudative or exudative effusions 1

Recommended Diagnostic Algorithm

1. Start with Light's criteria as the primary diagnostic tool for all pleural effusions requiring thoracentesis 3
2. If Light's criteria suggest exudate but heart failure is suspected clinically, apply SEAG or NT-proBNP to reclassify 3
3. If liver failure is suspected with ambiguous Light's criteria, use the pleural fluid/serum albumin ratio to confirm hepatic hydrothorax 3
4. Consider "concordant exudates" (where both protein ratio >0.5 AND pleural fluid LDH >160 IU/L) as having higher predictive value than meeting only one criterion 6