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, are used to classify pleural effusions as transudative or exudative. That distinction matters because transudates usually point to a systemic problem such as heart failure or cirrhosis, while exudates more often suggest local pleural disease such as infection, malignancy, or inflammation.
The three criteria compare pleural fluid with serum: pleural protein ratio, pleural LDH ratio, and pleural LDH relative to the upper limit of normal serum LDH. If any one criterion is met, the effusion is classified as exudative.
The calculator also shows the serum-pleural albumin gradient and pleural cholesterol, which can help when Light's Criteria overcall exudates in patients with heart failure on diuretics.
Accurate effusion classification matters because the next step differs depending on whether the fluid is transudative or exudative. That is why the criteria are used as an initial branching point rather than as a standalone diagnosis.
The albumin gradient is especially helpful when a patient with heart failure or another systemic cause has been diuresed and the fluid looks more exudative than it really is.
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.
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This calculator applies the classic Light's criteria exactly as an either-or rule: an effusion is classified as exudative if any one of the three standard criteria is met. It also shows supportive secondary markers such as the serum-pleural albumin gradient for the common scenario where diuresed heart-failure effusions are misclassified as exudates.
The result is a fluid-classification aid rather than a final etiology diagnosis. Pleural pH, cell count, culture, cytology, ADA, and the clinical setting still drive the actual workup once an effusion has been classified.
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 treatment of the underlying condition, consider repeating the sample or checking additional markers to confirm the cause and guide symptom management.