What are Light's criteria for distinguishing transudative from exudative pleural effusions?

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Last updated: November 3, 2026 • View editorial policy

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Light's Criteria for Pleural Effusion Classification

A pleural effusion is classified as an exudate if it meets ANY ONE of the following three criteria: pleural fluid protein/serum protein ratio >0.5, pleural fluid LDH/serum LDH ratio >0.6, or pleural fluid LDH >2/3 (67%) of the upper limit of normal serum LDH. 1, 2

The Three Criteria

Light's criteria require simultaneous measurement of both pleural fluid and serum samples to calculate:

  • Pleural fluid to serum protein ratio >0.5
  • Pleural fluid to serum LDH ratio >0.6
  • Pleural fluid LDH >0.67 times the upper limit of normal serum LDH

Meeting just one criterion classifies the effusion as exudative 2.

Performance Characteristics

Light's criteria demonstrate excellent sensitivity (98%) but moderate specificity (72%) for identifying exudates 2. This design intentionally prioritizes not missing serious exudative causes like malignancy or infection, even at the cost of misclassifying some transudates as exudates. The positive likelihood ratio is 3.5 and negative likelihood ratio is 0.03 2.

The most important clinical pitfall is the 25-30% misclassification rate of cardiac and hepatic transudates as exudates 2. This occurs particularly in heart failure patients receiving diuretics, where fluid reabsorption concentrates pleural proteins and LDH.

Correcting Misclassified Transudates

When Light's criteria suggest an exudate but clinical suspicion strongly favors heart failure or cirrhosis:

For Heart Failure:

  • Calculate serum albumin minus pleural fluid albumin (albumin gradient)
  • If >1.2 g/dL, correctly reclassifies ~80% of false exudates as transudates 2, 3
  • Alternatively, NT-proBNP >1500 μg/mL in serum or pleural fluid accurately identifies heart failure (sensitivity 92-94%, specificity 88-91%) 2

For Cirrhosis:

  • Calculate pleural fluid albumin/serum albumin ratio
  • If <0.6, confirms hepatic hydrothorax when Light's criteria are ambiguous 2, 3

When Serum Samples Are Unavailable

If venipuncture cannot be performed, use pleural fluid-only parameters:

Either pleural fluid LDH >67% upper limit of normal serum LDH OR pleural fluid cholesterol >55 mg/dL has diagnostic accuracy equivalent to Light's criteria 2, 4. Recent validation in over 7,000 patients confirms a triple combination (protein >3 g/dL, LDH >250 IU/L, or cholesterol >55 mg/dL) matches Light's criteria performance with 98% sensitivity 4.

Clinical Application Algorithm

  1. Obtain pleural fluid and serum samples simultaneously
  2. Apply Light's criteria—if ANY criterion positive, classify as exudate
  3. If exudate classification but high clinical suspicion for heart failure:
    • Calculate albumin gradient (>1.2 g/dL reclassifies as transudate)
    • Or measure NT-proBNP (>1500 μg/mL confirms cardiac cause)
  4. If exudate classification but high suspicion for cirrhosis:
    • Calculate pleural fluid/serum albumin ratio (<0.6 confirms hepatic hydrothorax)
  5. If serum unavailable, use pleural fluid LDH >67% upper normal limit or cholesterol >55 mg/dL

The key principle: Light's criteria were designed to avoid missing serious exudative causes (malignancy, infection, tuberculosis), accepting some false-positive exudates that can be corrected with albumin measurements when clinical context suggests transudate 2.

References

2
Guideline

ers statement on benign pleural effusions in adults.

European Respiratory Journal, 2024

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