Effect of Loop Diuretics on the Fractional Excretion of Urea in Decompensated Heart Failure

Fractional excretion of urea (FEUrea) is often used to understand the etiology of acute kidney injury (AKI) in patients receiving diuretics. Although FEUrea demonstrates diagnostic superiority over fractional excretion of sodium (FENa), clinicians often assume FEUrea is not affected by diuretics. To assess the intravenous loop diuretic effect on FEUrea. We analyzed a prospective cohort (n=297) hospitalized with hypervolemic heart failure at Yale New Haven Hospital System. FENa and FEUrea were calculated at baseline and serially after diuretics. The change in FEUrea at peak diuresis was compared with the pre-diuretic baseline. Mean baseline FEUrea was 35.2% ± 10.5% and increased by a mean 5.6% ± 10.5% following 80 mg (40–160 mg) of furosemide equivalents (P < .001). The magnitude of change in FEUrea was clinically important as the distribution of change in FEUrea was similar to the overall distribution of baseline FEUrea. Change in FEUrea was related to the diuretic response (r = 0.61, P < .001), with a larger FEUrea increase in diuretic responders (8.8%, interquartile range [IQR]: 1.8–16.9) than non-responders (1.2%, IQR: −3.2 to 5.5; P < .001). Diuretic administration reclassified 27% of patients between low and high FEUrea groups across a 35% threshold. Neither change in FEUrea nor percentage reclassified out of a low FEUrea category differed between patients with and without AKI (P > .63 for both). FEUrea is meaningfully affected by loop diuretics. The degree of change in FEUrea is highly variable between patients and commonly of a magnitude that could reclassify across categories of FEUrea.