FEUrea Calculator (Fractional Excretion of Urea)
Fractional excretion of urea from urine and serum urea and creatinine — the diuretic-resistant alternative to FENa for separating prerenal AKI (< 35%) from acute tubular necrosis (> 50%).
How it works
FEUrea (%) = (urine urea × plasma creatinine) ÷ (plasma urea × urine creatinine) × 100; < 35% prerenal, > 50% ATN
Urea is reabsorbed mainly in the proximal tubule, upstream of where loop and thiazide diuretics act, so its fractional excretion stays low in volume-depleted patients even when a dose of furosemide has flooded the urine with sodium. That makes FEUrea the preferred index in the many hospital patients whose kidney injury appears after diuretics — heart failure and cirrhosis especially. Below 35% the kidney is conserving urea because perfusion is low; above 50% the tubules are damaged and reabsorb little. In the study by Carvounis it correctly classified over 90% of patients on diuretics compared with about 50% for FENa. Like all urinary indices it is a snapshot that can change within hours and should be read alongside the history and fluid balance.
Worked example
Urine urea 180, serum urea 14, urine creatinine 7,500, serum creatinine 165: FEUrea = (180 × 165) / (14 × 7,500) × 100 = 28% — prerenal, despite recent diuretics.
Frequently asked questions
How do I convert BUN to urea?
Urea (mmol/L) = BUN (mg/dL) × 0.357; the same factor applies to urine urea nitrogen. Because urea appears in both numerator and denominator, using mg/dL for both is also fine.
Should I order FENa and FEUrea together?
Yes when the diuretic history is uncertain — a low FEUrea with a high FENa strongly suggests a diuretic effect masking prerenal physiology.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.