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.
Transudate vs. Exudate Etiology
Transudates
Exudates
CLINICAL INSIGHT
How it Works
Standard Light's Criteria (Serum-Dependent)
Pleural Fluid Protein : Serum Protein ratio > 0.5
Pleural Fluid LDH : Serum LDH ratio > 0.6
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.
CLINICAL INSIGHT
Practical Pearls
The Diuretic Challenge
Diuretics in CHF patients can 'concentrate' pleural fluid, causing a transudate to meet exudative criteria (pseudo-exudate).
Albumin & Protein Gradients
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.
CLINICAL INSIGHT
Evidence Base
The 2026 Validation (Triple Rule)
Revisiting Light's criteria: a validated blood-free triple combination matches diagnostic accuracy in over 7000 patients.
Porcel JM, Porcel L, Palma R, Bielsa S.ERJ Open Research2026
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.