FEUrea Calculator — Acute Kidney Injury Cause Differentiation

Fractional Excretion of Urea (FEUrea) is determined by comparing the clearance of urea to the clearance of creatinine in the kidneys. The core mathematical formula is expressed as:

FEUrea (%) = ((Urine Urea * Serum Creatinine) / (Urine Creatinine * Serum Urea)) * 100

In this equation, serum and urine values must share identical units of measurement (typically mg/dL or mmol/L). A calculated FEUrea value strictly below 35% strongly points toward a prerenal etiology where the kidneys are retaining sodium and water due to hypoperfusion, whereas a value above 50% generally indicates intrinsic renal damage such as acute tubular necrosis.

Consider a patient undergoing evaluation for a sudden rise in serum creatinine. The laboratory results return the following values: Serum Creatinine = 1.5 mg/dL, Serum Urea = 45 mg/dL, Urine Creatinine = 90 mg/dL, and Urine Urea = 900 mg/dL. To find the FEUrea, we substitute these numbers into the formula: First, multiply Urine Urea by Serum Creatinine (900 * 1.5 = 1350). Next, multiply Urine Creatinine by Serum Urea (90 * 45 = 4050). Divide the first product by the second (1350 / 4050 = 0.3333). Finally, multiply by 100 to convert to a percentage, resulting in an FEUrea of 33.33%. Because this value falls below 35%, it strongly suggests a prerenal cause for the acute kidney injury.

Always ensure that serum and urine chemistry panels are drawn concurrently to avoid temporal discrepancies caused by rapid clinical interventions or fluid shifts. Be mindful that diuretic administration can skew standard fractional excretion of sodium (FENa) values into misleading ranges, making FEUrea a superior choice because urea handling is less influenced by loop or thiazide diuretics. Always interpret calculated renal indices within the complete context of the patient's physical examination, fluid balance, and clinical history.