FENa / FEUrea
Compute the fractional excretion of sodium (FENa, Espinel 1976) and the fractional excretion of urea (FEUrea, Carvounis 2002) from a paired urine and serum sample.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Urine sodium
- 20 mEq/L
- Plasma sodium
- 140 mEq/L
- Urine creatinine
- 50 mg/dL
- Plasma creatinine
- 2.0 mg/dL
- Urine urea
- 300 mg/dL
- Plasma urea
- 60 mg/dL
Result: FENa ~0.57% (prerenal); FEUrea ~20% (prerenal).
The tool opens with these values already filled in. Replace them with your own.
What you enter
- Urine sodium
- Plasma sodium
- Urine creatinine
- Plasma creatinine
- Urine urea
- Plasma urea (BUN)
What this is
FENa < 1% classically suggests a prerenal pattern and FENa > 2% an intrinsic tubular pattern; FEUrea < 35% suggests prerenal physiology and remains interpretable when the patient is on a loop or thiazide diuretic, which invalidates FENa. Sophie Well returns both fractions side-by-side with the conventional band labels.
When to use it
Use this in the workup of acute kidney injury when the volume question is genuinely ambiguous and the answer will change management - i.e., before committing to aggressive fluids, diuresis, or a renal-replacement decision. Both indices are screening numbers, not diagnoses: glomerular disease, contrast nephropathy, rhabdomyolysis, and early obstruction can all sit at FENa < 1% despite intrinsic injury, and chronic kidney disease blunts both numbers. Read FENa/FEUrea alongside the urine sediment, the BUN:Cr ratio, the bladder-scan / Foley output, and the bedside volume exam before acting on either threshold.
How this is calculated
Espinel CH. The FENa test. JAMA. 1976;236(6):579-581. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea. Kidney Int. 2002;62(6):2223-2229. Read the sources: Espinel 1976 (FENa) ↗, Carvounis 2002 (FEUrea) ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.
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