What are Light's criteria for classifying pleural effusions as exudative versus transudative?

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

• Light's criteria require simultaneous collection of both pleural fluid and serum samples to measure protein and LDH levels. 1

• Meeting just one of the three thresholds is sufficient to label the effusion exudative; meeting none indicates a transudate. 1

• The criteria were designed to maximize detection of exudates (high sensitivity) to avoid missing serious conditions like malignancy or infection. 2

Performance Characteristics

• Light's criteria demonstrate 98% sensitivity for identifying exudative effusions, meaning they rarely miss true exudates. 1, 3

• The specificity is only 72–80%, indicating frequent false-positive labeling of transudates as exudates. 1, 3

• Positive likelihood ratio = 3.5; negative likelihood ratio = 0.03. 1

• Approximately 25–30% of cardiac or hepatic transudates are misclassified as exudates, especially in patients receiving diuretics. 1, 3

Critical Pitfall: Diuretic-Induced Misclassification

• Diuretic therapy is the most common cause of misclassification, concentrating pleural fluid and making transudates appear exudative. 1

• The British Thoracic Society advises avoiding bilateral thoracentesis in patients with clinically obvious heart failure unless atypical features are present or the effusion fails to respond to diuretic therapy. 1

Re-classification Tools for Suspected "False Exudates"

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

Serum-Effusion Albumin Gradient (SEAG)

• Calculate as serum albumin minus pleural fluid albumin. 1

• SEAG >1.2 g/dL reclassifies approximately 80% of false-positive exudates as transudates with 97.5% accuracy. 1, 4

• An alternative albumin ratio (pleural fluid/serum albumin <0.6) also identifies transudates. 1

• SEAG remains reliable in diuretic-treated patients, whereas the protein ratio correctly diagnoses only approximately 66% of cases under the same conditions. 1

NT-proBNP for Heart Failure Confirmation

• Pleural fluid or serum NT-proBNP >1500 pg/mL confirms heart-failure-related effusion. 1

• Diagnostic performance: serum NT-proBNP 92% sensitivity, 88% specificity; pleural fluid NT-proBNP 94% sensitivity, 91% specificity. 1

• Positive likelihood ratio ≈ 10.9, negative likelihood ratio ≈ 0.07. 1

Alternative Approach When Serum Is Unavailable

• 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

• Combination of pleural fluid cholesterol and LDH achieves 98% accuracy, 98% sensitivity, and 95% specificity without requiring serum samples. 5

Additional Laboratory Pitfalls

• Analytical platform variability can cause up to 18% discordance in classification between different laboratory analyzers, particularly affecting LDH measurements. 1, 6

• CT attenuation values and ultrasound echogenicity cannot reliably differentiate transudates from exudates, showing approximately 69% sensitivity and 66% specificity for CT. 1

• Certain conditions (e.g., non-expansile lung, chylothorax, superior vena cava syndrome) may produce either transudative or exudative effusions, limiting reliance on a single biochemical rule. 1

Clinical Management Algorithm

1. Apply Light's criteria to all pleural effusions requiring diagnostic thoracentesis (send simultaneous serum and pleural fluid for protein and LDH). 1, 2

2. If exudate by Light's criteria but clinical picture suggests heart failure or cirrhosis (especially if on diuretics):

   • Calculate SEAG: if >1.2 g/dL, reclassify as transudate. 1, 4
   • Measure NT-proBNP (serum or pleural fluid): if >1500 pg/mL, confirm heart failure etiology. 1

3. If transudate confirmed, >80% are due to heart failure and can be managed with continued diuretic therapy without further invasive testing. 1

4. If true exudate, proceed with additional diagnostic work-up (cytology, microbiology, imaging) to identify malignancy, infection, or other treatable causes. 1, 2