Light's criteria distinguish exudative from transudative pleural effusions. Which additional marker, when ADDED to Light's criteria, is used to reclassify borderline exudates as transudates in heart failure patients on diuretics?

• A Pleural fluid cholesterol > 60 mg/dL
• B Pleural fluid NT-proBNP > 1500 pg/mL
• C Pleural fluid pH < 7.20
• D Serum-to-pleural fluid albumin gradient > 1.2 g/dL ✓

Explanation

Light's criteria misclassify up to 25% of transudates as exudates in patients on diuretic therapy (diuretics concentrate pleural fluid proteins). A serum-to-pleural albumin gradient >1.2 g/dL (or serum-to-pleural TP gradient >3.1 g/dL) strongly suggests a true transudate, allowing reclassification. Pleural NT-proBNP elevation confirms cardiac origin but doesn't reclassify Light's criteria. Pleural cholesterol >60 mg/dL supports exudate in cholesterol analysis. pH <7.20 suggests complicated parapneumonic or malignant effusion.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.