Use Light's Criteria to classify pleural effusions as transudative or exudative. Calculates protein ratio, LDH ratio, and albumin gradient for differential diagnosis.
Light’s Criteria, established by Dr. Richard Light in 1972, remain the gold standard for classifying pleural effusions as transudative or exudative. This fundamental distinction determines the diagnostic and therapeutic approach: transudates are caused by systemic conditions (heart failure, cirrhosis, nephrosis) and typically resolve with treatment of the underlying disease, while exudates result from local pleural disease (infection, malignancy, inflammation) and require further investigation.
The three Light’s criteria compare pleural fluid to serum values: (1) pleural fluid protein/serum protein ratio >0.5, (2) pleural fluid LDH/serum LDH ratio >0.6, and (3) pleural fluid LDH greater than 2/3 the upper limit of normal serum LDH. Meeting ANY one criterion classifies the effusion as exudative.
This calculator also computes the serum-pleural albumin gradient and pleural cholesterol, which can help reclassify misidentified exudates in patients with heart failure on diuretics — a common clinical scenario where Light’s criteria may overcall exudates. Check the example with realistic values before reporting.
Accurate effusion classification is critical because the management pathways diverge completely. A transudative effusion from heart failure needs diuresis, not invasive procedures. An exudative effusion from empyema needs antibiotics and drainage, not just medical management.
Light’s Criteria misclassify approximately 25% of transudates as exudates, particularly in diuresed heart failure patients. The albumin gradient helps correct these false positives, making comprehensive analysis essential.
Light’s Criteria (ANY one = exudate): 1. Pleural protein / Serum protein > 0.5 2. Pleural LDH / Serum LDH > 0.6 3. Pleural LDH > 2/3 × serum LDH upper limit of normal Additional: Albumin gradient = Serum albumin − Pleural albumin ≥1.2 g/dL → likely transudate <1.2 g/dL → supports exudate Cholesterol >45 mg/dL → supports exudate
Result: Exudative — 2 of 3 criteria met
Protein ratio 0.57 (>0.5) and LDH ratio 1.25 (>0.6) both meet exudative criteria. This effusion requires further workup: cell count, culture, cytology, pH, and possibly adenosine deaminase (ADA) for tuberculosis screening.
Approximately 25-30% of heart failure patients on diuretics will have pleural fluid that meets Light’s criteria for exudate due to concentration of protein and LDH by fluid reabsorption. The serum-pleural albumin gradient (≥1.2 g/dL favors transudate), pleural fluid cholesterol (≤45 mg/dL favors transudate), and pleural fluid NT-proBNP (>1500 pg/mL indicates cardiac origin) are the best tools for correcting this misclassification.
For confirmed exudates, the differential is narrowed by additional tests: cell count and differential (neutrophils suggest acute infection; lymphocytes suggest TB or malignancy; eosinophils suggest air/blood in space), cytology (60% sensitivity for malignancy), bacterial culture, AFB stain/culture, ADA for TB, amylase for pancreatitis/esophageal rupture, and triglycerides (>110 mg/dL for chylothorax).
Hepatothorax (bilious fluid from biliary-pleural fistula), urinothorax (creatinine-rich transudative effusion from obstructive uropathy), and chylothorax (milky lymphocyte-rich triglyceride-rich effusion from thoracic duct injury) are rare but important diagnoses made by specific pleural fluid characteristics.
This is the classic "misclassified transudate" scenario, common in heart failure patients on diuretics. Check the serum-pleural albumin gradient: if ≥1.2 g/dL, the effusion is likely transudative despite meeting Light’s criteria. NT-proBNP >1500 pg/mL in pleural fluid also supports a cardiac transudate.
Light’s criteria have ~98% sensitivity for exudates (very few exudates are missed) but only ~75% specificity (25% of transudates are misclassified as exudates). This high sensitivity/lower specificity design was intentional — it is safer to over-investigate a transudate than to miss an exudate.
Pleural fluid pH <7.2 suggests complicated parapneumonic effusion or empyema requiring tube drainage. pH <7.2 in the absence of infection may indicate malignancy, rheumatoid pleurisy, or esophageal rupture. Always collect pleural fluid for pH in a heparinized blood gas syringe and analyze immediately.
Adenosine deaminase (ADA) >40 IU/L has high sensitivity (~90%) and specificity (~90%) for tuberculous pleurisy in areas of moderate-to-high TB prevalence. In low-prevalence areas, the positive predictive value drops. ADA is less useful in immunocompromised patients and empyema (where ADA can be elevated for other reasons).
The main differential for a lymphocytic exudative effusion is tuberculosis and malignancy. In developed countries, malignancy is more common; in TB-endemic areas, TB pleurisy predominates. Other causes include lymphoma, sarcoidosis, and chronic rheumatoid pleurisy. Cytology detects malignancy in ~60% of cases; repeat thoracentesis increases yield to ~75%.
If a transudative effusion recurs despite optimal treatment of the underlying condition (maximal diuresis for CHF), consider: (1) diagnostic repeat thoracentesis to exclude a new exudative cause, (2) pleural fluid NT-proBNP to confirm cardiac etiology, (3) indwelling pleural catheter for symptom management, or (4) pleurodesis in selected cases. Use this as a practical reminder before finalizing the result.