What are Light's criteria for diagnosing pleural effusion?

Light's Criteria for Diagnosing Pleural Effusion

Light's criteria classify a pleural effusion as an exudate 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 (or >2/3) of the upper limit of normal serum LDH. 1, 2, 3

Diagnostic Performance

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

• Sensitivity: 98% for detecting exudative effusions 1, 2, 3
• Specificity: 72-80% for excluding transudates 2, 4
• Positive likelihood ratio: 3.5 1, 2
• Negative likelihood ratio: 0.03 1, 2

The criteria were intentionally designed to maximize exudate detection to avoid missing serious conditions like malignancy or infection, which explains the high sensitivity at the expense of specificity. 1

Clinical Application Algorithm

Step 1: Obtain Required Samples

• Measure both pleural fluid and serum samples for protein and LDH levels 1, 2
• If serum samples are unavailable, you can use pleural fluid LDH >67% of upper normal serum limit or pleural fluid cholesterol >55 mg/dL as alternatives 3, 5

Step 2: Apply Light's Criteria

Calculate the three ratios and determine if any one criterion is met:

1. Pleural fluid/serum protein ratio >0.5
2. Pleural fluid/serum LDH ratio >0.6
3. Pleural fluid LDH >0.67 of upper normal serum LDH 1, 2, 3

• If ≥1 criterion met → Classify as exudate
• If 0 criteria met → Classify as transudate 2

Common Pitfall: False Exudates

Light's criteria misclassify approximately 25-30% of cardiac and hepatic transudates as exudates, particularly in patients on diuretics. 3, 4 When clinical suspicion suggests a transudate but Light's criteria indicate an exudate, use these corrective measures:

Reclassification Tools (in order of accuracy):

1. Serum-effusion albumin gradient (SEAG) >1.2 g/dL → Indicates transudate with 97.5% accuracy 2, 4
2. Pleural fluid/serum albumin ratio <0.6 → Indicates transudate 2
3. NT-proBNP >1500 μg/mL (pleural fluid or serum) → Confirms heart failure etiology with positive LR 10.9 2
   • Serum NT-proBNP: 92% sensitivity, 88% specificity 2
   • Pleural fluid NT-proBNP: 94% sensitivity, 91% specificity 2

The SEAG method demonstrates the highest efficacy for diagnosing transudates (accuracy 97.5%) and effectively identifies transudates misclassified by Light's criteria. 4

Abbreviated Approach When Serum Unavailable

If venipuncture cannot be performed, the combination of pleural fluid protein and LDH alone has discriminative capacity equivalent to standard Light's criteria, with 95.4% sensitivity and 80.2% specificity. 1, 5

Critical Caveats

• Analytical platform variability: Different laboratory analyzers show 18% discordance in pleural effusion classification, primarily due to LDH assay performance differences between serum and pleural fluid 6
• Imaging cannot substitute: CT attenuation values, ultrasound echogenicity, and effusion size cannot replace biochemical analysis using Light's criteria 2
• Bilateral effusions in obvious heart failure: Do not aspirate unless atypical features present or failure to respond to therapy 1