What are the Lights criteria (Light's criteria) for differentiating between transudative and exudative pleural effusions?

Light's 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 of the upper limit of normal serum value. 1

The Three Criteria

Light's criteria require simultaneous measurement of both pleural fluid and serum samples for the following parameters:

• Pleural fluid to serum protein ratio >0.5 1
• Pleural fluid to serum LDH ratio >0.6 1
• Pleural fluid LDH >0.67 (or 67%) of the upper limit of normal serum LDH 1

Meeting any single criterion classifies the effusion as exudative. 1

Performance Characteristics

Light's criteria demonstrate excellent sensitivity (98%) but lower specificity (72%) for identifying exudative effusions. 1, 2 This translates to a positive likelihood ratio of 3.5 and a negative likelihood ratio of 0.03. 1, 2

The criteria were deliberately designed to maximize detection of exudates to avoid missing potentially serious conditions such as malignancy or infection. 1 This design philosophy explains why the specificity is intentionally lower—the goal is to err on the side of caution by over-identifying exudates rather than missing them. 1

Critical Pitfall: Pseudoexudates

The most important limitation is that Light's criteria misclassify approximately 25-30% of transudates (particularly from heart failure and cirrhosis) as exudates, creating "pseudoexudates." 1, 3, 4 This occurs most commonly in patients on diuretics. 4

How to Identify and Correct Pseudoexudates

When Light's criteria suggest an exudate but clinical suspicion is high for heart failure or liver disease, use these corrective measures:

• Serum-pleural fluid albumin gradient >1.2 g/dL correctly reclassifies approximately 80% of pseudoexudates as transudates. 5, 3
• Pleural fluid to serum albumin ratio <0.6 confirms hepatic hydrothorax when Light's criteria are ambiguous. 1
• NT-proBNP >1500 μg/mL (in either pleural fluid or serum) identifies heart failure with 92-94% sensitivity and 88-91% specificity. 5, 3

Alternative Approach When Serum Unavailable

If serum samples cannot be obtained, use pleural fluid LDH >67% of the upper limit of normal OR pleural fluid cholesterol >55 mg/dL—this combination has discriminative capacity equivalent to Light's criteria. 1, 3

Clinical Application Algorithm

1. Obtain simultaneous pleural fluid and serum samples for protein and LDH measurement 1

2. Apply Light's criteria: If any one criterion is met, classify as exudate 1

3. If exudate is identified but clinical picture suggests heart failure or cirrhosis (especially if patient is on diuretics):

   • Calculate serum-pleural fluid albumin gradient 5, 3
   • If gradient >1.2 g/dL, reclassify as transudate 5, 3
   • Consider NT-proBNP measurement if heart failure suspected 5, 3

4. If transudate confirmed: Treat underlying heart failure or cirrhosis; further invasive testing usually unnecessary 1

5. If exudate confirmed: Proceed with additional diagnostic workup for pneumonia, malignancy, tuberculosis, or pulmonary embolism 1, 6

Additional Caveats

Be aware that different laboratory analyzers can produce discordant results in up to 18% of cases, particularly affecting LDH measurements in pleural fluid. 7 This analytical variability is most problematic when pleural fluid protein is in the borderline range of 25-35 g/L. 7

Some conditions can produce either transudates or exudates (such as non-expansile lung, chylothorax, and superior vena cava syndrome), so classification alone does not always indicate specific etiology. 1