What are the current Light’s criteria, including newer parameters like the serum‑effusion albumin gradient and pleural fluid cholesterol, for distinguishing exudative from transudative pleural effusions?

Light's Criteria and Newer Parameters for Pleural Effusion Classification

Core Light's Criteria

A pleural effusion is classified as an exudate if it meets at least one of the following three thresholds: 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

Performance Characteristics

• Sensitivity: 98% for identifying exudates 1
• Specificity: 72% for identifying exudates 1
• Positive likelihood ratio: 3.5; negative likelihood ratio: 0.03 1
• Meeting none of the three criteria classifies the effusion as a transudate 1

Critical Limitation

• 25-30% of cardiac or hepatic transudates are misclassified as exudates, particularly in patients receiving diuretic therapy 1, 2
• Diuretic therapy is the most common cause of misclassification by concentrating pleural fluid 1

Newer Parameters for Reclassification

Serum-Effusion Albumin Gradient (SEAG)

When Light's criteria suggest an exudate but clinical suspicion points to a transudate (especially heart failure or cirrhosis), use SEAG >1.2 g/dL to reclassify the effusion as a transudate with 97.5% accuracy. 1

• SEAG is calculated as: serum albumin minus pleural fluid albumin 1
• Reclassifies approximately 80% of "false" exudates correctly 1
• Demonstrated highest efficacy for diagnosing transudates (accuracy 97.50%) 3

Albumin Ratio Alternative

• Pleural fluid albumin ÷ serum albumin <0.6 indicates a transudate 1
• Can be used interchangeably with SEAG for reclassification 1

Pleural Fluid Cholesterol

When serum samples are unavailable, use pleural fluid LDH >67% of the upper limit of normal serum LDH AND pleural fluid cholesterol >55 mg/dL to classify as an exudate with discriminative capacity equivalent to Light's criteria. 1

• This "or" rule provides an alternative when simultaneous serum collection is not feasible 1, 4

NT-proBNP for Heart Failure Confirmation

For effusions suspected to be cardiac in origin, pleural fluid or serum NT-proBNP >1500 pg/mL confirms heart failure etiology. 1

• Pleural fluid NT-proBNP: sensitivity 94%, specificity 91% 1
• Serum NT-proBNP: sensitivity 92%, specificity 88% 1
• Positive likelihood ratio: 10.9; negative likelihood ratio: 0.07 1
• Serum NT-proBNP offers no advantage over pleural fluid levels, so either can be used 4

Algorithmic Approach

Step 1: Apply Light's Criteria

• Collect simultaneous pleural fluid and serum samples 1
• Measure protein and LDH in both 1, 4
• If any one criterion is met → classify as exudate 1
• If none are met → classify as transudate 1

Step 2: Evaluate for Pseudoexudate

When Light's criteria indicate exudate BUT clinical context strongly suggests transudate (heart failure, cirrhosis, diuretic use):

• Calculate SEAG: if >1.2 g/dL → reclassify as transudate 1, 5
• OR calculate albumin ratio: if <0.6 → reclassify as transudate 1
• Consider NT-proBNP if heart failure suspected: if >1500 pg/mL → confirms cardiac etiology 1

Step 3: When Serum Unavailable

• Use pleural fluid LDH >67% upper normal limit AND cholesterol >55 mg/dL 1

Important Caveats and Pitfalls

Analytical Variability

• Up to 18% discordance in classification can occur between different laboratory analyzers, particularly affecting LDH measurements 1, 6
• LDH assays may perform differently in serum versus pleural fluid 6

Clinical Contexts Requiring Caution

• Diuretic therapy is the leading cause of false-positive exudates 1, 5
• Traumatic pleural taps and coronary artery bypass grafting can also produce pseudoexudates 5
• Certain conditions (non-expansile lung, chylothorax, superior vena cava syndrome) may produce either type 1

What NOT to Rely On

• Imaging alone cannot replace biochemical analysis 1
• CT attenuation values show significant overlap (sensitivity 69%, specificity 66%) 1
• Ultrasound echogenicity patterns are unreliable for transudate-exudate discrimination 1
• Effusion size is not definitive, though massive effusions typically suggest malignancy 1