FeUrea Calculator

FeUrea Calculator for Nephrology. In the derivation cohort, a FEUrea of 35% or less pointed to prerenal physiology (preserved tubular avidity), whereas values above 35%, and especially near 50-60%, indicated ATN. In that study, prerenal patients averaged roughly 25-28% regardless of diuretic use, while ATN patients averaged about 59%; 89% of diuretic-treated prerenal patients had a FEUrea below 35% even though only 48% of them had a FENa below 1%. A low FEUrea should prompt a trial of volume optimization and correction of the underlying hemodynamic insult, while a high FEUrea supports supportive management of established tubular injury. Treat the 35% cutoff as a guide, not an absolute rule, and always weigh it against urine sediment and the clinical trajectory.

How this calculator works

The fractional excretion of urea (FEUrea) expresses the percentage of filtered urea that is ultimately excreted in the urine, computed as (urine urea x plasma creatinine) / (plasma urea x urine creatinine) x 100. Because urea reabsorption in the proximal tubule is driven largely by passive solvent drag rather than by active sodium-avid transport, FEUrea reflects tubular avidity for water and solute while remaining relatively insensitive to loop and thiazide diuretics, which blunt sodium reabsorption and artificially raise the fractional excretion of sodium (FENa). Only a spot urine and paired serum sample are needed; no timed collection is required. Note that "urea" here is measured as BUN in the United States, but the ratio is unit-independent as long as urea/BUN and creatinine are each expressed in matching units in numerator and denominator.

When to use this calculator

Use FEUrea to distinguish prerenal azotemia from acute tubular necrosis (ATN) in a patient with oliguric or non-oliguric acute kidney injury, particularly when the patient has received diuretics and FENa is therefore unreliable. It is most informative early in AKI before therapy or resuscitation has altered tubular handling. It should not be used in patients with chronic kidney disease at baseline, obstructive uropathy, glomerulonephritis, or contrast/pigment nephropathy, and it is uninterpretable in the setting of osmotic diuresis, high-protein feeding, gastrointestinal bleeding, or corticosteroid/catabolic states that independently shift urea handling. It does not apply once the differential has been resolved by other means such as urine microscopy.

Inputs used

  • Urine urea nitrogen
  • Serum urea nitrogen
  • Urine creatinine
  • Serum creatinine

Clinical interpretation

In the derivation cohort, a FEUrea of 35% or less pointed to prerenal physiology (preserved tubular avidity), whereas values above 35%, and especially near 50-60%, indicated ATN. In that study, prerenal patients averaged roughly 25-28% regardless of diuretic use, while ATN patients averaged about 59%; 89% of diuretic-treated prerenal patients had a FEUrea below 35% even though only 48% of them had a FENa below 1%. A low FEUrea should prompt a trial of volume optimization and correction of the underlying hemodynamic insult, while a high FEUrea supports supportive management of established tubular injury. Treat the 35% cutoff as a guide, not an absolute rule, and always weigh it against urine sediment and the clinical trajectory.

Worked example

Consider a diuretic-treated patient with serum urea (BUN) 60 mg/dL, serum creatinine 2.5 mg/dL, urine urea 300 mg/dL, and urine creatinine 40 mg/dL. FEUrea = (300 x 2.5) / (60 x 40) x 100 = 750 / 2400 x 100 = 31.25%. A value of about 31% is below the 35% threshold, favoring prerenal azotemia despite furosemide exposure that might have driven FENa above 1% and produced a misleadingly "renal" pattern.

Limitations and safety notes

FEUrea's superiority over FENa is not consistently reproduced: several later cohorts and a systematic review found that FEUrea did not reliably outperform FENa and sometimes misclassified patients, so it should not be used as a stand-alone arbiter. Urea handling is confounded by any state that alters urea load or ADH-driven reabsorption, including gastrointestinal bleeding, high-protein or catabolic states, corticosteroids, sepsis, and osmotic diuresis, all of which can push FEUrea into the prerenal range even during ATN or vice versa. It performs poorly in CKD, mixed AKI, and in patients already resuscitated or on renal-replacement therapy, and a single spot value cannot capture a rapidly evolving process.

Frequently asked questions

Why use FEUrea instead of FENa in a patient on diuretics?

Loop and thiazide diuretics block sodium reabsorption and raise FENa, which can falsely suggest ATN in a truly prerenal patient. Urea is reabsorbed mostly by passive forces in the proximal tubule and is far less affected by these agents, so FEUrea stays low in prerenal azotemia even after diuretic exposure.

What FEUrea cutoff separates prerenal AKI from ATN?

A FEUrea of 35% or less favors prerenal azotemia, while values above 35% (typically 50-65% in the derivation data) favor acute tubular necrosis. This threshold comes from the original Carvounis validation and is a decision aid, not a definitive test.

Do I use BUN or urea in the formula?

Either works as long as you are consistent. In US units the serum and urine values are BUN; elsewhere they are urea in mmol/L. Because FEUrea is a ratio, the units cancel provided the same measure (urea or BUN) is used in both numerator and denominator alongside creatinine.

When is FEUrea unreliable?

Avoid it in chronic kidney disease, obstruction, glomerulonephritis, and after fluid resuscitation. It is also confounded by GI bleeding, high-protein feeding, corticosteroids, catabolic or septic states, and osmotic diuresis, since these independently alter urea handling. Later studies show it does not always beat FENa, so combine it with urine microscopy and clinical context.

Is FEUrea validated as better than FENa?

The original 2002 study found it more sensitive and specific, especially with diuretics, but subsequent cohorts and a systematic review did not consistently confirm this advantage. Treat it as a complementary index rather than a replacement for FENa and clinical judgment.

References

  • Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002. PMID: 12427149.
  • Kanbay M, Kasapoglu B, Perazella MA. Acute tubular necrosis and pre-renal acute kidney injury: utility of urine microscopy in their evaluation - a systematic review. Int Urol Nephrol. 2009. PMID: 19921458.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: April 24, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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