How can I apply Light's criteria (pleural fluid protein/serum protein > 0.5, pleural fluid lactate dehydrogenase/serum lactate dehydrogenase > 0.6, or pleural fluid lactate dehydrogenase > 2/3 of the upper limit of normal) to distinguish an exudative from a transudative pleural effusion, and what adjunctive tests help when diuretic therapy or heart failure may cause false‑positive exudates?

Light's Criteria for Pleural Effusion Classification

Primary Diagnostic Approach

Apply Light's criteria by measuring simultaneous pleural fluid and serum samples: classify as exudate if ANY ONE of these is met: pleural fluid/serum protein ratio >0.5, pleural fluid/serum LDH ratio >0.6, or pleural fluid LDH >2/3 the upper limit of normal serum LDH. 1, 2

Performance Characteristics

• Sensitivity: 97.5–98% for identifying exudates 2, 3
• Specificity: 70–80% for identifying exudates 2, 3
• Positive likelihood ratio: 3.5; Negative likelihood ratio: 0.03 2
• Meeting none of the three criteria indicates a transudate 2

Critical Limitation

Light's criteria misclassify approximately 25–30% of cardiac and hepatic transudates as exudates, particularly in patients receiving diuretics 2, 3, 4. This is the most common pitfall in clinical practice.

---

Correcting False-Positive Exudates

When Light's criteria suggest exudate but clinical context (heart failure, cirrhosis, diuretic therapy) points to transudate, apply these adjunctive tests in order:

For Suspected Heart Failure

1. Serum-effusion albumin gradient (SEAG): serum albumin minus pleural fluid albumin >1.2 g/dL reclassifies ~80–83% of false exudates as transudates with 97.5% accuracy 2, 5, 4

2. Pleural fluid/serum albumin ratio <0.6 indicates transudate (78% accuracy for cardiac effusions) 2, 4

3. NT-proBNP: serum or pleural fluid >1500 pg/mL confirms heart failure with 92–94% sensitivity and 88–91% specificity 2

   • Serum NT-proBNP is preferred; pleural fluid measurement offers no advantage 6

For Suspected Hepatic Hydrothorax

• Pleural fluid/serum albumin ratio <0.6 has 77–78% accuracy for correctly identifying liver-related transudates misclassified by Light's criteria 2, 4

Clinical Scoring Alternative

• Porcel clinical scoring model ≥7 points indicates cardiac etiology with 92% accuracy (positive LR 12.7, negative LR 0.39) 2

---

Alternative When Serum Unavailable

If venipuncture is not feasible, use pleural fluid LDH >67% of upper limit of normal serum LDH AND pleural fluid cholesterol >55 mg/dL as an alternative with comparable discriminative capacity to Light's criteria 2, 7. However, this is an unusual circumstance and simultaneous serum sampling remains the standard.

---

Essential Practical Considerations

Sample Collection

• Collect pleural fluid and serum simultaneously to ensure accurate ratio calculations 2
• Send all samples for protein, LDH, pH, glucose, cytology, Gram stain, AFB stain, and culture in sterile vials plus blood culture bottles 6

Laboratory Variability Pitfall

• Up to 18% discordance in classification can occur between different laboratory analyzers, particularly affecting LDH measurements 2, 8
• LDH assays may perform differently in serum versus pleural fluid 8

Clinical Context Overrides

• Do not aspirate bilateral effusions in clinically obvious transudates (e.g., symmetric effusions with clear heart failure) unless atypical features present or failure to respond to therapy 6
• Diuretic therapy is the single most common cause of transudate misclassification as exudate 2

---

Diagnostic Algorithm Summary

1. Apply Light's criteria first (98% sensitivity for exudates) 1, 2, 3
2. If exudate by Light's but clinical picture suggests transudate:
   • Heart failure suspected → SEAG >1.2 g/dL or NT-proBNP >1500 pg/mL 2
   • Cirrhosis suspected → Pleural fluid/serum albumin ratio <0.6 2, 4
3. If transudate by Light's criteria: no further biochemical testing needed for classification 2

This approach maximizes detection of serious exudative causes (malignancy, infection) while providing tools to rescue the 25–30% of transudates incorrectly labeled as exudates 2, 3, 4.