Fractional Excretion of Urea
FEUrea helps differentiate prerenal azotaemia from intrinsic renal injury, retaining accuracy even after diuretic administration. Enter serum and urine laboratory values below.
How to Use This Calculator
Collect paired serum and urine labs
Draw samples as close together as possible in time. FEUrea is especially useful in diuretic-treated patients with AKI.
Choose consistent units
Enter BUN/urea in mg/dL or mmol/L and creatinine in mg/dL or µmol/L. The calculator converts automatically.
Interpret thresholds carefully
FEUrea <35% indicates prerenal azotaemia; >50% suggests intrinsic renal injury. Integrate with urine microscopy and haemodynamic assessment.
Formula
FEUrea (%) = (Urine urea × Serum creatinine ÷ (Serum urea × Urine creatinine)) × 100
When serum values are reported as BUN, use the same units for urine urea to maintain consistency.
Full Description
Fractional excretion of urea is less affected by loop diuretics than FENa, making it useful in patients with suspected prerenal azotaemia who recently received diuretics. FEUrea values below 35% indicate avid tubular reabsorption typical of prerenal states, whereas higher values signal intrinsic tubular injury. Always interpret FEUrea alongside clinical findings, urine output, and other laboratory data.
Frequently Asked Questions
Why use FEUrea instead of FENa?
Loop diuretics increase urinary sodium, making FENa unreliable. FEUrea remains low in prerenal states despite diuretics, offering better diagnostic discrimination.
Can FEUrea be used in CKD?
Baseline tubular dysfunction may limit accuracy, but FEUrea can still aid differentiation when interpreted with the broader clinical picture.
Do different laboratories report urea differently?
Yes. Some report BUN (mg/dL) while others report urea (mmol/L). The calculator allows both and converts automatically.
What if urine urea is unavailable?
FEUrea cannot be calculated without urine urea. Consider FENa or alternative biomarkers and repeat labs if necessary.