What are Light's criteria and how are they used to differentiate exudative from transudative pleural effusions?

Light's Criteria for Pleural Effusion Classification

Definition and Application

Light's criteria classify a pleural effusion as an exudate if ANY ONE of the following three thresholds is met: 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

• You must obtain simultaneous pleural fluid and serum samples to measure protein and LDH for proper application 1
• Meeting just one criterion labels the effusion exudative; meeting none indicates a transudate 1
• The criteria were deliberately designed to maximize exudate detection (high sensitivity) to avoid missing serious conditions like malignancy or infection 2

Performance Characteristics

Light's criteria demonstrate 98% sensitivity but only 72% specificity for identifying exudates 1, 2:

• Positive likelihood ratio = 3.5; negative likelihood ratio = 0.03 1
• This means the test is excellent at ruling out exudates when negative, but generates many false-positive exudates 1
• Approximately 25–30% of cardiac or hepatic transudates are misclassified as exudates, especially in patients receiving diuretics 1, 3

Correcting Misclassification: The Albumin Gradient Approach

When Light's criteria suggest an exudate but clinical suspicion points strongly to heart failure or cirrhosis, use these reclassification tools:

Primary Method: Serum-Effusion Albumin Gradient (SEAG)

• Calculate serum albumin minus 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 back to transudates 1
• SEAG remains reliable even in diuretic-treated patients, whereas the protein ratio correctly diagnoses only 66% of cases under the same conditions 1

Alternative: Albumin Ratio

• Pleural fluid albumin ÷ serum albumin <0.6 indicates a transudate 1

Combined Gradient Method (Spanish Guidelines)

• The combination of SEAG >1.2 g/dL AND serum-pleural protein gradient >3.1 g/dL correctly identifies 100% of pseudoexudates in heart failure and 99% in hepatic hydrothorax 3

NT-proBNP for Heart Failure Confirmation

When heart failure is suspected as the cause of a "false exudate":

• Pleural fluid or serum NT-proBNP >1500 pg/mL confirms heart failure etiology 1, 3
• 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 over serum levels 2

Alternative Approach When Serum Is Unavailable

If you cannot obtain a simultaneous serum sample, use this "or" rule with comparable discriminative capacity to standard Light's criteria 1, 5:

• Pleural fluid LDH >67% of upper limit of normal serum LDH 1
• AND pleural fluid cholesterol >55 mg/dL 1, 3

Critical Pitfalls and Limitations

Diuretic Therapy

• Diuretic use is the most common cause of misclassification, concentrating pleural fluid and making transudates appear exudative 1
• Always consider SEAG or NT-proBNP in patients on diuretics with suspected heart failure 1

Laboratory Platform Variability

• Different analytical platforms can cause up to 18% discordance in classification, particularly affecting LDH measurements 1, 6
• This occurs because assays may perform differently in serum versus pleural fluid 6

Imaging Cannot Replace Biochemical Analysis

• CT attenuation values show significant overlap between transudates and exudates (69% sensitivity, 66% specificity) 1
• Ultrasound echogenicity patterns are unreliable for transudate-exudate discrimination 1
• Even advanced imaging (CT, ultrasound, MRI) cannot replace biochemical analysis 1

Special Circumstances

• Certain conditions (non-expansile lung, chylothorax, superior vena cava syndrome) may produce either transudative or exudative effusions, limiting reliance on a single biochemical rule 1

Clinical Management Algorithm

Step 1: Apply Light's criteria to all pleural effusions requiring diagnostic thoracentesis 1, 2

Step 2: If exudate criteria are met BUT clinical picture strongly suggests heart failure or cirrhosis (especially if on diuretics):

• Calculate SEAG 1, 4
• If SEAG >1.2 g/dL → reclassify as transudate and manage the underlying condition 1
• Consider NT-proBNP if diagnosis remains uncertain 1

Step 3: If correctly classified as transudate:

• >80% are due to heart failure and can be managed with diuretic therapy without further invasive testing 1
• Liver cirrhosis accounts for approximately 10% 1

Step 4: If confirmed as true exudate:

• Proceed with additional diagnostic workup: cytology (60% sensitivity for malignancy), microbiology (Gram stain, AFB stain, culture in sterile vials AND blood culture bottles), pH measurement if infection suspected 2
• Consider image-guided pleural biopsy or thoracoscopy if initial tests are non-diagnostic 2