How this instrument works
FEUrea works the same way FENa does — comparing how much of a filtered load actually leaves in urine against how much the tubule reclaims, corrected by creatinine — except the substance being tracked is urea nitrogen instead of sodium. Urine urea nitrogen and plasma urea nitrogen are both expressed in the same units, so no conversion factor belongs anywhere in the ratio; the units cancel cleanly on their own, and inserting an extra scaling step would only introduce an error that isn't there in the original arithmetic.
Carvounis, Nisar and Guro-Razuman proposed the test in a 2002 paper, built specifically for patients already taking diuretics, a group where FENa becomes unreliable because diuretics force sodium excretion regardless of the kidney's underlying state. Urea handling is comparatively less disturbed by that same diuretic effect, which is exactly why substituting it into the ratio restores a usable signal. Their paper set the cutoffs below 35% for a prerenal pattern and above 50% for acute tubular necrosis.
The trade-off is that urea reabsorption isn't perfectly passive either — it's influenced by urine flow rate and by the same hormonal state, antidiuretic hormone in particular, that governs water handling — so FEUrea is a substitute for the diuretic blind spot specifically, not a universally superior version of the older test. In a patient not on diuretics, FENa remains the more established first choice.
- Enter Urine urea nitrogen (Uurea) and Plasma urea nitrogen / BUN (Purea), both in mg/dL.
- Enter Plasma creatinine (Pcr) and Urine creatinine (Ucr), both in mg/dL.
- Read FEUrea as a percentage — below 35% leans prerenal, above 50% leans toward tubular injury.
Worked example — Uurea 300, Pcr 1.5, Purea 30, Ucr 50
Multiply the top: 300 × 1.5 = 450. Multiply the bottom: 30 × 50 = 1500. Divide: 450 ÷ 1500 = 0.30, and ×100 gives a FEUrea of exactly 30% — under the 35% cutoff, a prerenal pattern, and notably a reading FENa couldn't have offered honestly if this patient were also on a diuretic.
Compare a tubular-injury reading: Uurea 400, Pcr 1.8, Purea 35, Ucr 40. Top: 400 × 1.8 = 720. Bottom: 35 × 40 = 1400. Divide: 720 ÷ 1400 ≈ 0.514, or about 51.4% — just past the 50% cutoff for acute tubular necrosis, even in a patient whose diuretic use would have made FENa unreadable.
Questions
What FEUrea suggests reduced blood flow to the kidney?
Below 35%, per the 2002 paper by Carvounis, Nisar and Guro-Razuman that proposed the test — a pattern read the same way as a low FENa, but derived from nitrogen handling rather than sodium.
What FEUrea suggests tubular injury?
Above 50%, from that same paper — the cutoff proposed for acute tubular necrosis, sitting well clear of the prerenal range below 35%.
Why was FEUrea developed when FENa already existed?
Because diuretics make FENa unreliable by forcing sodium excretion regardless of the kidney's true state, and clinicians needed a version of the same logic built around a substance diuretics disturb less. Urea fit that role.
Do I need to convert units between urine and plasma urea nitrogen?
No. Both values are reported in mg/dL of nitrogen, so the units cancel directly in the ratio — no factor of two, or any other conversion, belongs anywhere in this calculation. Adding one would introduce an error rather than correct one.
Is FEUrea more accurate than FENa overall?
Not universally — it solves one specific blind spot. Urea reabsorption responds to urine flow rate and antidiuretic hormone in ways that can still shift the result, so FEUrea is best reserved for patients on diuretics rather than treated as a flatly superior replacement for FENa in every case.
Can FEUrea be used alongside FENa?
Yes, and in a patient not on diuretics there's little reason to reach for FEUrea over the better-established FENa. The two are complementary tools aimed at the same underlying question, chosen based on what medication history rules one of them out.
References
- Carvounis CP et al. 2002, Kidney Int — original description (PubMed)
- Renal Function Tests — StatPearls (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.