Light's Criteria: Gold standard for differentiating pleural exudates from transudates. ≥ 1 of 3 criteria met = exudate. Sensitivity 98%, Specificity 83%.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Mandatory first step in the evaluation of any new pleural effusion via diagnostic thoracentesis.
Differentiating systemic hydrostatic/oncotic pressure issues (Transudates) from local pleural disease (Exudates).
Screening for malignant pleural effusion (MPE) or complicated parapneumonic effusions.
Pleural Fluid LDH > 2/3 the Upper Limit of Normal (ULN) Serum LDH
The 2026 'Triple Combination' (Blood-Free)
Validated in >7,000 patients (Porcel et al., 2026). If blood sampling is unavailable, an effusion is classified as an EXUDATE if it meets ANY of these PF-only criteria:
1. PF Protein > 3 g/dL
2. PF LDH > 250 IU/L
3. PF Cholesterol > 55 mg/dL
Interpretation Rule
Light's Criteria: Any 1 positive = Exudate (Sensitivity 98%, Specificity ~71%). Triple Combination: Matches accuracy of Light's and reclassifies ~20% of false-positive transudates.
Section 3
Pearls/Pitfalls
The Diuretic Challenge
Diuretics in CHF patients can 'concentrate' pleural fluid, causing a transudate to meet exudative criteria (pseudo-exudate).
Albumin & Protein Gradients
Serum-Effusion Protein Gradient: (Serum Protein - Pleural Protein) > 3.1 g/dL = Transudate
Serum-Effusion Albumin Gradient (SEAG): (Serum Albumin - Pleural Albumin) > 1.2 g/dL = Transudate
Malignancy Pearls (CEA Interaction)
An LDH ratio > 0.6 is independently associated with false-negative CEA results in malignant effusions (Yang et al., 2025).
CEA > 10 ng/mL is highly suggestive of MPE, but 40% of MPEs are CEA-negative.
Internet-active patients often prefer allografts in surgical contexts—similarly, in medicine, patients may request 'blood-free' testing (Triple Rule) to avoid repeat venipuncture.
Section 4
Evidence Appraisal
The 2026 Validation (Triple Rule)
Revisiting Light's criteria: a validated blood-free triple combination matches diagnostic accuracy in over 7000 patients.
Porcel JM et al. • ERJ Open Research. 2026;Large-scale study (n=7280) proving that a PF-only combination of protein, LDH, and cholesterol matches Light's accuracy while improving specificity for transudates.
Effects of Light’s criteria on the diagnostic accuracy of pleural fluid carcinoembryonic antigen concentrations for malignant pleural effusion.
Yang DN et al. • Scientific Reports. 2025;Analyzed the positive correlation between CEA and LDH/Protein levels; found that LDH ratio >0.6 predicts false-negative CEA in MPE.
Pleural effusions: the diagnostic separation of transudates and exudates.
Light RW et al. • Annals of Internal Medicine. 1972;The seminal paper that established the protein and LDH ratios as the clinical standard for the last 50+ years.
Before 1972, the differentiation of effusions was inconsistent, often relying solely on protein levels (>3.0 g/dL). Dr. Richard Light introduced the multi-parameter approach to capture the enzymatic activity (LDH) of inflammatory processes.
