Fractional Excretion of Urea (FEUrea) Calculator
Calculate FEUrea to differentiate prerenal azotemia from intrinsic renal AKI. Enter serum and urine urea and creatinine for instant clinical interpretation.
About This Calculator
The Fractional Excretion of Urea (FEUrea) Calculator is a clinical nephrology tool that helps healthcare professionals differentiate between prerenal azotemia and intrinsic renal causes of acute kidney injury (AKI). By comparing serum and urine concentrations of urea and creatinine, FEUrea provides valuable insight into whether the kidneys are appropriately conserving urea (prerenal physiology) or have lost this ability (intrinsic tubular damage).
The FEUrea formula calculates: FEUrea = (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100. The urinary flow rate cancels out in the derivation, making a spot urine sample sufficient for calculation. A FEUrea below 35% suggests prerenal causes of AKI such as volume depletion, decreased effective circulating volume from heart failure or cirrhosis, or hepatorenal syndrome. A value above 50% indicates intrinsic renal disease, most commonly acute tubular necrosis (ATN), with the kidneys unable to reabsorb urea appropriately. Values between 35-50% are considered indeterminate and require clinical correlation.
Why FEUrea over FENa: The fractional excretion of urea has largely supplanted FENa in many clinical settings because it is less affected by diuretic use. Loop diuretics and thiazides alter sodium reabsorption, invalidating FENa but having minimal effect on urea handling. FEUrea is also more reliable in patients with chronic kidney disease, glucosuria, radiocontrast-induced AKI, and those with underlying congestive heart failure or liver disease.
Regional considerations: The FEUrea formula and interpretation thresholds (<35% prerenal, 35-50% indeterminate, >50% intrinsic) are universally accepted across nephrology practice in India, the United States, the United Kingdom, and other regions. Units cancel out in the formula, so whether creatinine is reported in mg/dL or µmol/L, the result is identical. Normal reference ranges for serum urea (8-20 mg/dL or 2.9-7.1 mmol/L) and creatinine (0.6-1.2 mg/dL or 53-106 µmol/L) are consistent globally with minor inter-laboratory variation.
Frequently Asked Questions
What is FEUrea?
FEUrea (fractional excretion of urea) is a clinical parameter used to differentiate prerenal azotemia from intrinsic renal causes of acute kidney injury (AKI). It measures the percentage of urea filtered by the kidneys that is excreted in urine. FEUrea is generally preferred over FENa in patients taking diuretics.
How is FEUrea calculated?
FEUrea is calculated using the formula: FEUrea = (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100. The units cancel out in the ratio, so any consistent units (mg/dL or mmol/L) can be used for all parameters.
What do FEUrea results mean?
FEUrea below 35% suggests prerenal azotemia where the kidneys are appropriately conserving urea due to decreased perfusion from volume depletion, heart failure, or cirrhosis. FEUrea above 50% suggests intrinsic renal disease, most commonly acute tubular necrosis (ATN). Values between 35-50% are indeterminate and require clinical correlation.
When is FEUrea preferred over FENa?
FEUrea is preferred over FENa (fractional excretion of sodium) in patients taking diuretics, as diuretics alter renal sodium handling and invalidate FENa results. FEUrea is also useful in patients with chronic kidney disease, glucosuria, radiocontrast nephropathy, and AKI with underlying heart or liver failure.
What are normal ranges for serum urea and creatinine?
Normal serum urea (BUN) ranges from 8-20 mg/dL (2.9-7.1 mmol/L). Normal serum creatinine ranges from 0.7-1.3 mg/dL (62-115 µmol/L) in adults. Reference ranges may vary slightly between laboratories in the US, UK, and India.
Can FEUrea differentiate all causes of AKI?
No, FEUrea cannot differentiate all causes of AKI alone. It primarily helps distinguish prerenal azotemia from intrinsic renal disease. FEUrea in the 35-50% range is indeterminate. Clinical assessment including history, physical examination, urine microscopy, renal ultrasound, and response to fluid challenge are essential for complete evaluation.
Is FEUrea used in pediatric AKI assessment?
FEUrea can be used in pediatric AKI assessment, but interpretation thresholds may differ. Neonates and young children have immature renal tubular function, which may affect urea handling. Consultation with a pediatric nephrologist is recommended for accurate interpretation in pediatric patients.