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, 2

Light's Criteria Parameters

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

• 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 1

Clinical Significance and Application

Purpose

• Primary purpose: Differentiate between exudative and transudative pleural effusions
• This differentiation is crucial for determining underlying etiology and guiding treatment approach

Clinical Algorithm for Using Light's Criteria:

1. Obtain both pleural fluid and serum samples

   • Measure protein and LDH in both samples
   • Apply Light's criteria

2. If transudate (no criteria met):

   • Focus on systemic causes (heart failure, cirrhosis, nephrotic syndrome)
   • Direct therapy toward the underlying condition
   • Further pleural investigations usually unnecessary 1, 2

3. If exudate (≥1 criteria met):

   • Pursue additional diagnostic testing:
     • Cytology (for malignancy)
     • Gram stain and culture (for infection)
     • pH and glucose (for empyema, rheumatoid disease)
     • Adenosine deaminase (for tuberculosis)
     • Consider thoracoscopy if diagnosis remains unclear 2, 3

Limitations and Refinements

Misclassification Issues

• 25-30% of transudates (especially from heart failure or cirrhosis patients on diuretics) may be misclassified as exudates 1, 2, 4

Additional Tests to Resolve Misclassification:

When Light's criteria suggest exudate but clinical suspicion for transudate is high:

• Serum-effusion albumin gradient (SEAG) > 1.2 g/dL suggests transudate despite meeting Light's criteria 1, 4
• Albumin ratio (pleural fluid/serum) < 0.6 suggests transudate 1
• NT-proBNP in pleural fluid or serum > 1500 μg/mL suggests heart failure as cause 1, 2
• Clinical scoring model for heart failure (score ≥7) can identify cardiac cause with 92% accuracy 1

Alternative When Serum Sample Unavailable

• Use "OR" rule: pleural fluid LDH > 67% of upper limit of normal serum LDH OR pleural fluid cholesterol > 55 mg/dL 1

Analytical Considerations

• Despite being ratios (which should minimize variability), up to 18% discordance between different analytical platforms has been reported 5
• Laboratories should be aware that assays may perform differently in serum versus pleural fluid 5

Beyond Light's Criteria

For exudative effusions, additional markers can help determine specific etiology:

• Pleural fluid CRP levels can differentiate parapneumonic effusions and empyema from tuberculous and malignant effusions (cut-off ≥47.4 mg/dL) 6
• Low pH (< 7.2) suggests pleural infection, rheumatoid arthritis, malignancy, or esophageal rupture 2
• Low glucose (< 3.3 mmol/L) indicates empyema, rheumatoid disease, tuberculosis, malignancy, or esophageal rupture 2

Light's criteria remain the cornerstone of pleural effusion evaluation despite their limitations, with newer biomarkers and clinical scoring systems serving as valuable adjuncts to improve diagnostic accuracy.