What are Light's (criteria) used for in differentiating pleural effusions?

Light's Criteria for Differentiating Pleural Effusions

Light's criteria are the gold standard for differentiating exudative from transudative pleural effusions, with 98% sensitivity and 72% specificity for identifying exudates, guiding subsequent diagnostic and treatment decisions. 1

Light's Criteria Definition

Light's criteria classify a pleural effusion as an exudate if ANY ONE of the following is met:

Criteria	Threshold
Pleural fluid to serum protein ratio	> 0.5
Pleural fluid to serum LDH ratio	> 0.6
Pleural fluid LDH	> 0.67 of the upper limit of normal serum LDH value

Clinical Application and Importance

Light's criteria serve as the critical first step in the diagnostic algorithm for pleural effusions:

1. Initial Classification: Distinguishes between:

   • Exudates: Caused by local inflammatory processes (infection, malignancy, inflammation)
   • Transudates: Caused by systemic factors altering hydrostatic/oncotic pressures (heart failure, cirrhosis, nephrotic syndrome)

2. Guides Further Workup:

   • For exudates: Additional testing for specific etiology (cytology, cultures, pH, glucose)
   • For transudates: Focus on treating underlying systemic condition

Limitations and Refinements

While highly sensitive for exudates (98%), Light's criteria have limitations:

• Misclassification of Transudates: About 20% of transudates (particularly in patients on diuretics with heart failure) may be misclassified as exudates 2

• Analytical Variability: Up to 18% discordance between different laboratory platforms, primarily due to variations in LDH assay performance in pleural fluid 3

Additional Tests for Misclassified Effusions

When clinical suspicion for transudate is high despite meeting exudative criteria by Light's criteria, consider:

• Serum-Effusion Albumin Gradient (SEAG):

  • SEAG > 1.2 g/dL suggests transudate
  • Shows highest accuracy (97.5%) for identifying transudates 2

• Albumin ratio (pleural fluid/serum) < 0.6 suggests transudate 1

• NT-proBNP in pleural fluid or serum > 1500 μg/mL helps identify heart failure as cause 1

Practical Approach

1. Perform thoracentesis and obtain both pleural fluid and serum samples
2. Apply Light's criteria as first-line classification method
3. If exudate: Proceed with specific testing based on clinical suspicion
   • Cytology for suspected malignancy
   • Gram stain, culture, pH, glucose for suspected infection
   • Consider pleural fluid CRP to help differentiate parapneumonic effusions (cut-off ≥47.4 mg/dL) from tuberculous or malignant effusions 4
4. If transudate: Direct therapy toward underlying condition
5. If discrepancy between clinical impression and Light's criteria, use SEAG to resolve

Common Pitfalls to Avoid

• Relying solely on protein levels: Using only pleural fluid protein without applying complete Light's criteria can lead to misclassification
• Ignoring clinical context: Always interpret Light's criteria in the context of the patient's clinical presentation
• Laboratory variability: Be aware that different analytical platforms may affect classification 3
• Not considering diuretic effect: Diuretic therapy can increase protein concentration in transudative effusions, leading to misclassification

The European Respiratory Society and American Thoracic Society both recommend using Light's criteria as the standard approach for initial classification of pleural effusions 1.