What are the Light criteria used for in diagnosing pleural effusions?

Light Criteria for Diagnosing Pleural Effusions

Light's criteria are used to differentiate exudative from transudative pleural effusions by measuring biochemical parameters in pleural fluid and serum, with a pleural effusion classified as an exudate if it meets at least one of three specific criteria. 1, 2

The Three Criteria

A pleural effusion is classified as an exudate if it meets at least one of the following:

• Pleural fluid to serum protein ratio > 0.5 1, 2, 3
• Pleural fluid to serum LDH ratio > 0.6 1, 2, 3
• Pleural fluid LDH > 0.67 (or 2/3) of the upper limit of normal serum value 1, 2, 3

If none of these criteria are met, the effusion is classified as a transudate. 2

Performance Characteristics

• Light's criteria demonstrate 98% sensitivity for identifying exudates, meaning they rarely miss an exudative effusion 1, 2, 3
• The specificity is only 72%, which means approximately 25-30% of transudates (particularly cardiac and hepatic) are incorrectly classified as exudates 1, 2, 3
• The positive likelihood ratio is 3.5 and negative likelihood ratio is 0.03 2, 3
• The criteria were intentionally designed to maximize exudate detection to avoid missing serious conditions like malignancy or infection 2

When Serum Samples Are Unavailable

If you cannot obtain serum samples, alternative approaches include:

• Pleural fluid LDH > 67% of the upper limit of normal serum LDH 1
• Pleural fluid cholesterol > 55 mg/dL to classify as exudate 1
• Combined use of pleural fluid protein and LDH alone has discriminative capacity equivalent to standard Light's criteria 2, 4

Correcting Misclassification of Transudates

When Light's criteria classify an effusion as exudate but clinical suspicion suggests transudate (especially in heart failure or cirrhosis patients on diuretics):

• Calculate the serum-pleural fluid albumin gradient: if > 1.2 g/dL, the effusion is likely a transudate despite meeting Light's criteria 3, 5, 6
• This albumin gradient correctly reclassifies approximately 80% of "false" exudates 3
• Measure NT-proBNP in pleural fluid (> 1500 μg/mL indicates heart failure): sensitivity 94%, specificity 91% 3
• Serum NT-proBNP also performs well: sensitivity 92%, specificity 88% 3

Clinical Implications and Diagnostic Algorithm

1. Obtain paired pleural fluid and serum samples for protein and LDH measurement 2, 3
2. Apply Light's criteria to classify as exudate or transudate 1, 2
3. If classified as exudate but clinical picture suggests transudate (heart failure on diuretics, cirrhosis), calculate albumin gradient or measure NT-proBNP 3, 5, 6
4. Exudative effusions require extensive investigation for malignancy, infection, inflammatory conditions, pulmonary embolism, or autoimmune disorders 2, 3
5. Transudative effusions are most commonly due to heart failure (>80%) or liver cirrhosis (10%) 3

Important Caveats

• Analytical platform variability can cause 18% discordance in classification between different laboratory analyzers, particularly affecting LDH measurements 7
• The serum-effusion albumin gradient (SEAG) demonstrates 97.5% accuracy for identifying transudates, making it superior to Light's criteria for this purpose 5
• Abbreviated Light's criteria (omitting the pleural fluid/serum LDH ratio) maintain 95.4% sensitivity and 83.3% specificity when serum LDH is unavailable 4
• Biochemical analysis is essential—imaging studies (CT, ultrasound, MRI) cannot reliably differentiate transudates from exudates 3