What are Light's (Lymphatic fluid examination) criteria for classifying pleural effusion as exudative or transudative?

Light's Criteria for Classifying Pleural Effusions

Light's criteria are the gold standard for differentiating between exudative and transudative pleural effusions, with a pleural effusion classified as an exudate if it meets at least one of the following: pleural fluid to serum protein ratio >0.5, pleural fluid to serum LDH ratio >0.6, or pleural fluid LDH >0.67 of the upper limit of normal serum value. 1, 2

Diagnostic Algorithm for Pleural Effusion Classification

Step 1: Apply Light's Criteria

• Measure both pleural fluid and serum samples for:
  • Protein levels
  • Lactate dehydrogenase (LDH) levels 3

Step 2: Calculate the Following Ratios

• Pleural fluid protein/serum protein ratio
• Pleural fluid LDH/serum LDH ratio
• Pleural fluid LDH/upper limit of normal serum LDH 1, 2

Step 3: Interpret Results

• An effusion is classified as exudative if it meets at least one of the following:
  • Pleural fluid/serum protein ratio >0.5
  • Pleural fluid/serum LDH ratio >0.6
  • Pleural fluid LDH >2/3 (or >0.67) of the upper limit of normal serum LDH 1, 2
• If none of these criteria are met, the effusion is classified as transudative 3

Performance Characteristics

• Light's criteria have excellent sensitivity (98%) for identifying exudative effusions 1, 2
• Specificity is lower (72%), meaning some transudates may be misclassified as exudates 1, 2
• The positive likelihood ratio is 3.5 and the negative likelihood ratio is 0.03, indicating high reliability in ruling out exudative effusions when criteria are not met 2

Alternative Approaches When Serum Samples Are Unavailable

• Pleural fluid LDH >67% of the upper limit of normal serum LDH 1
• Pleural fluid cholesterol >55 mg/dL 1
• These alternatives have been shown to have discriminative capacity equivalent to Light's criteria 2

Clinical Implications and Pitfalls

• Common pitfall: Misclassification of transudates as exudates occurs in 25-30% of cases, particularly in patients with heart failure or cirrhosis who are on diuretics 1, 4
• Solution: Consider using Serum-Effusion Albumin Gradient (SEAG) as a complementary test when Light's criteria suggest exudate but clinical picture suggests transudate 4
• Accurate classification is crucial as it guides further diagnostic workup and management decisions 2
• Laboratory variability between different analytical platforms can affect classification in up to 18% of cases, particularly for pleural fluid with protein levels between 25-35 g/L 5

Additional Diagnostic Parameters

• Pleural fluid pH <7.2 suggests complicated parapneumonic effusion, empyema, rheumatoid arthritis, esophageal rupture, or malignancy 3
• Pleural glucose <3.3 mmol/L suggests empyema, rheumatoid disease, lupus, tuberculosis, malignancy, or esophageal rupture 3
• Elevated pleural fluid amylase (higher than serum or pleural/serum ratio >1.0) suggests pancreatitis, esophageal rupture, or malignancy 3

Clinical Significance

• Correct classification directs further investigation and management:
  • Transudates typically require treatment of the underlying condition (heart failure, cirrhosis, nephrosis) 6
  • Exudates require more extensive investigation for causes such as malignancy, infection, or inflammatory conditions 2
• In pleural infection, pH is a better discriminator than glucose for guiding management decisions 1