Light's criteria for exudative pleural effusion require at least one of: pleural LDH/serum LDH >0.6, pleural protein/serum protein >0.5, or pleural LDH >2/3 upper limit of serum normal. If a patient with heart failure on diuretics has an effusion meeting exudate criteria by Light's but clinical picture suggests transudate, which supplemental test helps confirm transudate?

• A Pleural fluid adenosine deaminase (ADA)
• B Pleural fluid cholesterol <45 mg/dL
• C Pleural fluid pH <7.2
• D Serum-pleural fluid albumin gradient >1.2 g/dL ✓

Explanation

Diuresis concentrates pleural LDH and protein, causing transudates to 'drift' into exudate territory by Light's criteria (misclassification in up to 25% of cardiac transudates on diuretics). The serum-pleural fluid albumin gradient >1.2 g/dL reliably identifies transudates in this scenario, as diuresis does not affect the albumin gradient. This test corrects the misclassification of Light's criteria. ADA is elevated in tuberculosis; pH <7.2 suggests empyema or malignancy; cholesterol is less reliable.

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

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Written and medically reviewed by the StethoPrep medical team.