How do you perform Light's criteria (Lactate Dehydrogenase) to diagnose the nature of a pleural effusion?

How to Perform Light's Criteria

Light's criteria classify a pleural effusion as an exudate if it meets at least one of three parameters: pleural fluid-to-serum protein ratio >0.5, pleural fluid-to-serum LDH ratio >0.6, or pleural fluid LDH >2/3 (0.67) of the upper limit of normal for serum LDH. 1, 2

Required Samples and Measurements

• Obtain both pleural fluid and serum samples simultaneously to measure protein and LDH levels in each 3
• Aspirate pleural fluid using a fine bore (21G) needle with a 50 ml syringe 3
• Send samples for protein and LDH analysis in both pleural fluid and serum 1

Applying the Three Criteria

Calculate the following three ratios and values:

1. Pleural fluid protein / Serum protein ratio - exudate if >0.5 1, 2
2. Pleural fluid LDH / Serum LDH ratio - exudate if >0.6 1, 2
3. Pleural fluid LDH absolute value - exudate if >0.67 (or 2/3) of the upper limit of normal for serum LDH 1, 2

If ANY ONE of these three criteria is met, classify the effusion as an exudate. 1, 4

When to Use Light's Criteria

• Apply Light's criteria when pleural fluid protein is borderline (between 25-35 g/L) to accurately differentiate exudates from transudates 3
• Use as the primary diagnostic tool for all pleural effusions requiring transudate versus exudate classification 4

Performance Characteristics

• Sensitivity: 97.5-98% for identifying exudates (excellent at ruling out transudates) 1, 4, 5
• Specificity: 70-80% (may misclassify some transudates as exudates, particularly in heart failure patients on diuretics) 1, 4, 5
• Positive likelihood ratio: 3.5; Negative likelihood ratio: 0.03 1, 2

Alternative Approach When Serum Sample Unavailable

If you cannot obtain a serum sample, use the "OR" rule combining pleural fluid LDH >67% of upper normal limit OR pleural fluid cholesterol >55 mg/dL, which has equivalent discriminative capacity to standard Light's criteria 1, 2, 5

Alternatively, isolated pleural fluid LDH >200 IU/L alone has 97.2% sensitivity and 95.6% overall accuracy for identifying exudates 6

Critical Pitfall: Pseudoexudates

Approximately 25% of cardiac and hepatic transudates are misclassified as exudates by Light's criteria, especially in patients receiving diuretics. 4, 7

When Light's criteria suggest exudate BUT clinical picture strongly suggests heart failure or cirrhosis:

• Calculate serum-effusion albumin gradient (SEAG): serum albumin minus pleural fluid albumin 2, 7
• If SEAG >1.2 g/dL, reclassify as transudate (this corrects ~80% of pseudoexudates) 2, 7
• Alternative: pleural fluid-to-serum albumin ratio <0.6 also indicates transudate 2
• SEAG has 97.5% accuracy for identifying true transudates 7

Additional Diagnostic Context

Once classified as exudate, measure pleural fluid pH in all non-purulent effusions if infection is suspected (pH ≤7.2 indicates complicated parapneumonic effusion requiring drainage) 3, 1

For suspected cardiac effusions, NT-proBNP >1500 μg/mL in either pleural fluid or serum has 92-94% sensitivity and 88-91% specificity 2