What it is
The fractional excretion of urea (FEUrea) is the percentage of filtered urea that ends up in the urine. Like FENa, it helps separate prerenal azotaemia (kidneys appropriately conserving) from intrinsic AKI / ATN (tubules failing to reabsorb). Its headline advantage: urea reabsorption is largely passive and follows water in the proximal tubule, so — unlike sodium handling — it is not disrupted by loop or thiazide diuretics. When a patient has had diuretics, FEUrea is the better discriminator.
Formula
$$ FEUrea,(%) = \frac{U_{urea} \times P_{Cr}}{BUN \times U_{Cr}} \times 100 $$
where $U_{urea}$ is urine urea nitrogen, $BUN$ is blood urea nitrogen, $U_{Cr}$ is urine creatinine, and $P_{Cr}$ is plasma creatinine. The calculator returns the percentage and a pattern category:
- < 35% → prerenal pattern
- 35–50% → indeterminate
- > 50% → intrinsic / ATN pattern
When to use it
- Differentiating prerenal AKI from ATN, especially when the patient is on diuretics and FENa is therefore unreliable.
- As a complement to the clinical assessment of volume status, urine output, and sediment — not a replacement for it.
- When the FENa result is borderline or implausible given a recent diuretic dose, and you need a second discriminator that the diuretic has not perturbed.
Worked example
Labs: urine urea 300 mg/dL, BUN 40 mg/dL, urine creatinine 80 mg/dL, plasma creatinine 1.5 mg/dL.
$$ FEUrea = \frac{300 \times 1.5}{40 \times 80} \times 100 = \frac{450}{3200} \times 100 \approx 14.1% $$
At 14.1%, well under the 35% threshold, this is a prerenal pattern — consistent with appropriate urea conservation in a volume-depleted patient, even if they have received furosemide.
Pitfalls
- Consistent units. Both urea values (urine urea and BUN) must be in the same units; mixing mg/dL with mmol/L corrupts the ratio.
- Pediatric data are thin. The <35% / >50% cut-offs come largely from adult cohorts. Interpret pediatric values alongside the whole clinical picture, not as a hard rule.
- Not a standalone test. Sepsis, contrast, CKD, high-protein states, and GI bleeding all shift urea independently of volume status. Use FEUrea as one input.
- Timing. A single spot value early in evolving AKI can mislead; correlate with the trajectory.
- Don’t reach for it when FENa already works. If the patient is diuretic-naïve, FENa is the classic discriminator; FEUrea earns its keep specifically when diuretics are on board.
Run it: [Fractional Excretion of Urea (FEUrea)](/calculator/feurea)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.