How do I calculate Light's (Light's) criteria for pleural effusion?

How to Calculate Light's Criteria for Pleural Effusion

Light's criteria classify a pleural effusion as an exudate if at least one of three specific ratios is met: pleural fluid to serum protein ratio >0.5, pleural fluid to serum LDH ratio >0.6, or pleural fluid LDH >0.67 (two-thirds) of the upper limit of normal serum value. 1

Required Laboratory Values

To calculate Light's criteria, you need the following measurements:

• Pleural fluid protein and serum protein (measured simultaneously) 1
• Pleural fluid LDH and serum LDH (measured simultaneously) 1
• Upper limit of normal for serum LDH from your laboratory's reference range 1

Step-by-Step Calculation

Calculate the following three ratios:

1. Protein ratio: Divide pleural fluid protein by serum protein 1

   • If >0.5 → suggests exudate

2. LDH ratio: Divide pleural fluid LDH by serum LDH 1

   • If >0.6 → suggests exudate

3. Absolute pleural fluid LDH: Compare to upper limit of normal serum LDH 1

   • If pleural fluid LDH >0.67 (or two-thirds) of the upper normal serum limit → suggests exudate

Interpretation

• If ANY ONE of these three criteria is met, classify the effusion as an exudate 1
• If NONE of the criteria are met, classify as a transudate 1

Performance Characteristics

• Light's criteria have excellent sensitivity (98%) for detecting exudates, meaning they rarely miss an exudative effusion 1
• However, specificity is lower (72%), meaning approximately 25-30% of transudates (especially cardiac and hepatic) may be misclassified as exudates 2
• The positive likelihood ratio is 3.5 and negative likelihood ratio is 0.03 1

Alternative When Serum Samples Unavailable

If you cannot obtain serum samples, use these pleural fluid-only criteria:

• Pleural fluid LDH >200 IU/L (or >67% of upper normal serum limit) 2, 3
• Pleural fluid cholesterol >55 mg/dL 2

Important Caveats

• Analytical platform variability: Different laboratory analyzers can produce discordant results in up to 18% of cases, particularly affecting LDH measurements 4
• Diuretic effect: Patients with heart failure on diuretics may have transudates misclassified as exudates by Light's criteria 5
• Borderline protein values: Light's criteria are most useful when pleural fluid protein is 25-35 g/L, where classification is uncertain 4
• Consider serum-effusion albumin gradient (SEAG): If Light's criteria suggest exudate but clinical picture suggests transudate (e.g., heart failure), SEAG has 97.5% accuracy for identifying transudates 5