How do I apply Light's criteria to determine whether a pleural effusion is an exudate or a transudate?

How to Apply Light's Criteria to Classify Pleural Effusions

Light's criteria classify a pleural effusion as an exudate if it meets ANY ONE of three thresholds: 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, 2

The Three Components of Light's Criteria

You need to obtain both pleural fluid and serum samples simultaneously and measure protein and LDH in both. 1 Calculate the following:

• Protein ratio: Divide pleural fluid protein by serum protein. If >0.5, the effusion is an exudate. 1, 3
• LDH ratio: Divide pleural fluid LDH by serum LDH. If >0.6, the effusion is an exudate. 1, 3
• Absolute pleural fluid LDH: Compare pleural fluid LDH to 0.67 times the upper limit of normal for serum LDH. If higher, the effusion is an exudate. 1, 3

Meeting just one criterion classifies the effusion as exudative; meeting none classifies it as transudative. 1, 3

Performance Characteristics

Light's criteria achieve 98% sensitivity but only 72% specificity for identifying exudates. 1, 4 This means:

• The criteria were deliberately designed to maximize detection of exudates to avoid missing serious conditions like malignancy or infection. 1, 2
• Approximately 25-30% of cardiac and hepatic transudates are misclassified as exudates, particularly in patients on diuretics. 1, 4
• The positive likelihood ratio is 3.5 and negative likelihood ratio is 0.03. 1, 3

When Serum Samples Are Unavailable

If you cannot obtain serum, use an "or" rule combining pleural fluid LDH >67% upper limit of normal serum LDH AND pleural fluid cholesterol >55 mg/dL, which has equivalent discriminative capacity to Light's criteria. 1

Correcting Misclassification in Suspected Transudates

When Light's criteria suggest exudate but you strongly suspect heart failure or cirrhosis (especially with borderline values near the cutoffs), apply these reclassification tools: 1, 4

• Serum-effusion albumin gradient (SEAG): Calculate serum albumin minus pleural fluid albumin. If >1.2 g/dL, the effusion is actually a transudate (correctly reclassifies ~80% of false exudates). 1, 3
• Albumin ratio: Calculate pleural fluid albumin divided by serum albumin. If <0.6, the effusion is a transudate. 1, 3
• NT-proBNP: Measure in either pleural fluid or serum. If >1500 pg/mL, this confirms heart failure as the cause (sensitivity 92-94%, specificity 88-91%). 1, 3

Critical Pitfalls to Avoid

• Diuretic therapy is the most common reason for misclassification, causing transudates to appear as exudates by concentrating pleural fluid. 1, 4
• Analytical platform variability can cause up to 18% discordance in classification between different laboratory analyzers, particularly affecting LDH measurements. 5
• Do not rely on imaging alone—CT attenuation values show significant overlap between transudates and exudates (sensitivity 69%, specificity 66%), and ultrasound echogenicity patterns are unreliable for differentiation. 3
• Some conditions (non-expansile lung, chylothorax, superior vena cava syndrome) can cause either transudate or exudate. 1

Clinical Context Matters

Once you classify the effusion:

• If transudate: Direct therapy toward underlying heart failure, cirrhosis, or nephrosis. 6, 7
• If exudate: Pursue further diagnostic workup for pneumonia, malignancy, tuberculosis, or pulmonary embolism, which account for most exudative effusions. 7
• Bilateral effusions in clinically obvious transudates should not be aspirated unless atypical features are present or there is failure to respond to therapy. 2