What are the Light criteria used for in differentiating between transudative and exudative pleural effusions?

Light Criteria for Pleural Effusion Differentiation

Light's criteria are used to differentiate exudative from transudative pleural effusions by measuring pleural fluid and serum protein and lactate dehydrogenase (LDH) levels, with an effusion classified as exudative if it meets at least one of three specific biochemical thresholds. 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. 1, 2

Performance Characteristics and Clinical Implications

Light's criteria have excellent sensitivity (98%) for detecting exudates but lower specificity (72%), meaning they are designed to avoid missing serious conditions like malignancy or infection at the expense of occasionally misclassifying transudates as exudates. 1, 2, 3

• The positive likelihood ratio is 3.5 and negative likelihood ratio is 0.03, indicating high confidence when criteria are met 1, 2
• Approximately 25-30% of cardiac and hepatic transudates may be incorrectly classified as exudates, particularly in patients on diuretics 3, 4

When Serum Samples Are Unavailable

If serum samples cannot be obtained, alternative approaches include: 3

• Pleural fluid LDH >67% of the upper limit of normal serum LDH 3
• Pleural fluid cholesterol >55 mg/dL 3

Correcting Misclassification

When Light's criteria suggest an exudate but clinical suspicion points to heart failure or cirrhosis, use the serum-albumin gradient to reclassify approximately 80% of "false" exudates: 2

• Serum albumin minus pleural fluid albumin >1.2 g/dL indicates a transudate 2
• Alternatively, pleural fluid/serum albumin ratio <0.6 suggests transudate 2
• NT-proBNP (serum >1500 μg/mL) has 92% sensitivity and 88% specificity for heart failure-related effusions 2

Clinical Decision Algorithm

1. Obtain pleural fluid and serum samples for protein and LDH 1, 5
2. Apply Light's criteria - if any criterion is met, classify as exudate 1, 2
3. If exudate: Pursue extensive workup for pneumonia, malignancy, tuberculosis, pulmonary embolism, or autoimmune disease 1, 5, 6
4. If transudate: Direct therapy toward underlying heart failure (>80% of transudates), cirrhosis (10%), or renal failure 2, 5
5. If exudate classification seems inconsistent with clinical picture: Calculate albumin gradient or measure NT-proBNP to identify misclassified transudates 2, 4

Critical Pitfalls to Avoid

• Never rely on imaging alone - ultrasound findings (echoes, septations) suggest exudate but cannot replace biochemical analysis 2
• Do not aspirate bilateral effusions in clinically obvious heart failure unless atypical features are present or therapy fails 1
• Biochemical analysis, not cytology or visual inspection, is the foundation of classification 2
• Measure pH immediately in all non-purulent effusions when parapneumonic effusion is suspected - pH ≤7.2 indicates need for chest tube drainage 1, 3