diuretic use will cause exudative effusion in heart failure patients

Heart failure patients undergoing diuretic treatment can have exudative pleural effusion. Elevated ProBNP (>1500) suggests heart failure related effusion. Serum-effusion protein difference > 3.1 g/dL or serum-effusion albumin difference > 1.2 g/dL also suggest true transudative effusion.

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Light's criteria are nearly 100% sensitive but only 83% specific for identifying exudates, and the specificity decreases in the setting of diuretic use.  Up to 20% of heart failure patients with pleural effusion may fulfill criteria for exudative effusion after receiving diuretics.  In this case, a serum-effusion protein difference >3.1 g/dL or serum-effusion albumin gradient >1.2 g/dL suggests a true transudative effusion.