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

Light Criteria for Pleural Effusion Classification

A pleural effusion is classified as an exudate if it meets at least one of the following three criteria: 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 value. 1, 2

Diagnostic Performance

The Light criteria are designed to maximize detection of exudates to avoid missing serious conditions like malignancy or infection:

• Sensitivity: 98% for identifying 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 means that if Light criteria are negative (transudate), you can be highly confident the effusion is truly transudative. The lower specificity means approximately 25-30% of transudates (particularly cardiac and hepatic) may be misclassified as exudates. 3

When Serum Samples Are Unavailable

If you cannot obtain serum samples, use these alternative criteria to classify as exudate: 3

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

Correcting Misclassification

When Light criteria incorrectly classify a transudate as an exudate (common in diuretic-treated heart failure patients), use these additional tests: 2

• Albumin gradient (serum albumin minus pleural fluid albumin) >1.2 g/dL correctly reclassifies approximately 80% of "false" exudates 2
• Albumin ratio (pleural fluid/serum albumin) <0.6 also helps reclassify false exudates 2
• NT-proBNP >1500 μg/mL in pleural fluid or serum identifies heart failure-related effusions with 92-94% sensitivity and 88-91% specificity 2

Critical Pitfalls to Avoid

• Never rely on imaging alone (CT, ultrasound, or MRI) to differentiate transudates from exudates—biochemical analysis is essential 2
• Never rely on visual appearance or microscopic examination alone—this leads to significant misclassification 2
• Be aware of analytical platform variability—different laboratory analyzers can produce discordant results in up to 18% of cases, particularly for LDH measurements in pleural fluid 4
• Remember that diuretic therapy in heart failure patients commonly causes transudates to be misclassified as exudates by Light criteria—use albumin gradient or NT-proBNP in these cases 2

Clinical Application Algorithm

1. Obtain paired pleural fluid and serum samples for protein and LDH measurement 1, 2
2. Calculate all three Light criteria ratios 1, 2
3. If ANY one criterion is met → classify as exudate and pursue extensive workup for malignancy, infection, pulmonary embolism, or inflammatory conditions 1, 2
4. If ALL three criteria are negative → classify as transudate and direct therapy toward heart failure, cirrhosis, nephrosis, or renal failure 2, 5
5. If transudate classification seems inconsistent with clinical picture (especially in diuretic-treated patients) → measure albumin gradient or NT-proBNP 2