What are Light’s criteria for distinguishing exudative versus transudative pleural effusions and how should they be applied in clinical management?

Light's Criteria for Pleural Effusion Classification

Definition and Application

A pleural effusion is classified as an exudate if it meets ANY ONE of the following three criteria: pleural fluid-to-serum protein ratio >0.5, pleural fluid-to-serum LDH ratio >0.6, or pleural fluid LDH >0.67 times the upper limit of normal serum LDH. 1

• Collect pleural fluid and serum samples simultaneously to measure protein and LDH for accurate ratio calculation 1
• Meeting just one threshold is sufficient to label the effusion exudative; meeting none indicates a transudate 1
• Light's criteria demonstrate 98% sensitivity but only 72% specificity for identifying exudates 1
• The positive likelihood ratio is 3.5 and negative likelihood ratio is 0.03 1

Performance and Limitations

Approximately 25–30% of cardiac or hepatic transudative effusions are misclassified as exudates, especially in patients receiving diuretics. 1

• Diuretic therapy is the most common cause of misclassification, concentrating pleural fluid and making transudates appear exudative 1
• Different laboratory analyzers can cause up to 18% discordance in classification, particularly affecting LDH measurements 2
• The criteria were intentionally designed with high sensitivity to avoid missing serious conditions like malignancy or infection 3

Correcting Misclassification: The Albumin Gradient Approach

When Light's criteria suggest an exudate but clinical suspicion strongly points to heart failure or cirrhosis, use the serum-effusion albumin gradient (SEAG) to reclassify. 1

SEAG Calculation and Interpretation:

• Calculate SEAG as: serum albumin − pleural fluid albumin 1
• SEAG >1.2 g/dL indicates a transudate with 97.5% accuracy 1, 4
• This reclassifies approximately 80% of "false" exudates as true transudates 1
• Alternative: pleural fluid/serum albumin ratio <0.6 also identifies transudates 1

Combined Gradient Approach (Highest Accuracy):

• SEAG >1.2 g/dL PLUS serum-pleural protein gradient (SPPG) >3.1 g/dL together identify 100% of pseudoexudates in heart failure and 99% in hepatic hydrothorax 5, 6

NT-proBNP for Heart Failure Confirmation

Pleural fluid or serum NT-proBNP >1500 pg/mL confirms heart failure etiology with high diagnostic accuracy. 1

• Serum NT-proBNP: 92% sensitivity, 88% specificity 1
• Pleural fluid NT-proBNP: 94% sensitivity, 91% specificity 1
• Positive likelihood ratio ≈10.9, negative likelihood ratio ≈0.07 1
• Serum measurement is sufficient; pleural fluid NT-proBNP offers no advantage 3

Alternative Approach When Serum Is Unavailable

If venipuncture cannot be performed, use pleural fluid LDH >67% of upper limit normal serum LDH AND pleural fluid cholesterol >55 mg/dL. 1, 5

• This "or" rule provides discriminative capacity comparable to standard Light's criteria 1, 7
• Pleural fluid cholesterol >55 mg/dL suggests true exudate 5

Clinical Management Algorithm

Step 1: Apply Light's Criteria

• Measure simultaneous serum and pleural fluid protein and LDH 1
• If ANY criterion is met → classify as exudate 1
• If NONE are met → classify as transudate 1

Step 2: Evaluate for Pseudoexudate (if exudate + high clinical suspicion for heart failure/cirrhosis)

• Check for diuretic use in the patient's medication history 1
• Calculate SEAG: if >1.2 g/dL → reclassify as transudate 1, 4
• For maximum accuracy, also calculate SPPG: if >3.1 g/dL → confirms transudate 5, 6
• Consider NT-proBNP if heart failure suspected: if >1500 pg/mL → confirms cardiac etiology 1

Step 3: Manage Based on Final Classification

• Transudates (>80% due to heart failure): Continue diuretic therapy without further invasive testing 1
• True exudates: Proceed with additional diagnostic workup (cytology, microbiology, pH, glucose, imaging) to identify malignancy, infection, or inflammatory conditions 1, 3

Essential Pleural Fluid Tests for All Exudates

Send every exudative effusion for protein, LDH, pH, glucose, cytology, Gram stain, acid-fast bacilli stain, and culture in both sterile vials AND blood culture bottles. 3

• Document gross appearance and odor immediately: purulent suggests empyema, milky suggests chylothorax, bloody raises suspicion for malignancy 3
• pH ≤7.2 indicates complicated parapneumonic effusion/empyema requiring chest tube drainage 3
• Glucose <3.3 mmol/L suggests complicated infection if pH unavailable 3
• Cytology detects only 60% of malignant effusions; if negative and malignancy suspected, obtain image-guided pleural biopsy or thoracoscopy 3
• Measure hematocrit if grossly bloody: >50% of peripheral blood hematocrit confirms hemothorax 3

Critical Pitfalls to Avoid

• Never rely on imaging alone: CT attenuation values show only 69% sensitivity and 66% specificity; ultrasound echogenicity is unreliable for transudate-exudate discrimination 1
• Always use blood culture bottles in addition to sterile vials for microbiology to maximize diagnostic yield 3
• Avoid blind pleural biopsies: use ultrasound- or CT-guided techniques or thoracoscopy instead 3
• Obtain contrast-enhanced CT with effusion still present before complete drainage to optimize pleural visualization and identify biopsy targets 3
• Do not aspirate bilateral effusions in clinically obvious transudates unless atypical features present or failure to respond to therapy 3
• Reconsider pulmonary embolism and tuberculosis in persistently undiagnosed effusions, as these have specific treatments 3