Fractional Excretion of Urea (FEUrea)

What FEUrea measures and why it emerged

FEUrea indicates what fraction of the urea filtered by the glomeruli is eliminated in the urine, using creatinine as the reference for filtration. It follows the same logic as the fractional excretion of sodium (FENa), with urea in place of sodium.

The reason for the swap is the diuretic. Carvounis, Nisar and Guro-Razuman, in Kidney International in 2002, pointed out that many patients with a prerenal cause receive diuretics, which reduce sodium reabsorption and raise FENa. Urea excretion, in contrast, depends mainly on passive forces and is less affected by these drugs.

The study prospectively evaluated 102 episodes of acute kidney injury, then called acute renal failure. Mean FEUrea was 27.9% in prerenal azotemia without diuretics, 24.5% in prerenal azotemia with diuretics and 58.6% in acute tubular necrosis. Since then, FEUrea has been used as support in the differential diagnosis of acute kidney injury, always together with clinical data.

How to use the calculator: inputs and result

The calculator asks for four blood and urine values collected at the same time:

• Serum urea, in mg/dL, greater than zero and up to 1,000. Example: 40.
• Urine urea, in mg/dL, greater than zero and up to 10,000. Example: 900.
• Serum creatinine, in mg/dL, greater than zero and up to 30. Example: 1.0.
• Urine creatinine, in mg/dL, greater than zero and up to 1,000. Example: 100.

Values outside these ranges produce a message saying which field to correct. The output is the FEUrea, in %, with two decimal places, and the corresponding range, read from the already rounded value:

• 35% or less: range associated with prerenal azotemia by Carvounis (2002), including after diuretics;
• above 35% and up to 50%: nondiscriminatory range;
• above 50%: range associated with an intrinsic cause, such as acute tubular necrosis.

A note reminds you that FEUrea is less affected by diuretics than FENa. With the field example values, the FEUrea is 22.50%.

FEUrea formula and worked example

FEUrea is calculated like FENa, replacing sodium with urea. Urine volume and time cancel out:

FEUrea (%) = (Urine urea × Serum creatinine) ÷ (Serum urea × Urine creatinine) × 100

The terms are concentrations in simultaneous samples: urine and serum urea in mg/dL, urine and serum creatinine in mg/dL.

Teaching example (not the interpretation of a patient): serum urea 80 mg/dL, urine urea 600 mg/dL, serum creatinine 2.0 mg/dL and urine creatinine 100 mg/dL.

1. Numerator: 600 × 2.0 = 1,200.
2. Denominator: 80 × 100 = 8,000.
3. FEUrea: 1,200 ÷ 8,000 × 100 = 15.00%.
4. Range: 35% or less.

Second case: serum urea 60 mg/dL, urine urea 700 mg/dL, serum creatinine 2.5 mg/dL and urine creatinine 60 mg/dL. The FEUrea is (700 × 2.5) ÷ (60 × 60) × 100 = 48.61%, and it falls in the nondiscriminatory range, above 35% and up to 50%. With serum urea 50, urine urea 1,100, serum creatinine 2.0 and urine creatinine 80, the FEUrea is 55.00%, in the above 50% range.

How to understand FEUrea: the 35% cutoff and the evidence

FEUrea is read against cutoffs, not against a normal range. In the Carvounis study, low FEUrea (35% or less) was more sensitive and specific than FENa for separating prerenal azotemia from acute tubular necrosis, especially after diuretics: 89% of prerenal patients on diuretics had FEUrea below 35%, while only 48% had FENa below 1%.

Later studies showed more modest performance. In the meta-analysis by Abdelhafez and colleagues, published in the American Journal of the Medical Sciences in 2024, with 11 studies and 1,108 hospitalized patients, FEUrea above 35% had a pooled sensitivity of 66% and specificity of 75% for distinguishing intrinsic from prerenal acute kidney injury.

In practice, FEUrea helps organize clinical reasoning, but it does not define the cause of kidney injury on its own. For that reason the calculator only states which cause each range was associated with, and the final interpretation belongs to the physician, with the clinical history, physical examination and the course of creatinine.

Urea or BUN, common errors and checklist

The most common unit question with FEUrea is the difference between urea and blood urea nitrogen (BUN). Brazilian laboratory reports usually give urea, and the North American literature uses BUN. Urea in mg/dL is about 2.14 times the BUN, which is the ratio between the molar mass of urea (60 g/mol) and that of the two nitrogen atoms it contains (28 g/mol).

In FEUrea this factor cancels out, as long as serum and urine urea are in the same form. The Carvounis study used urea nitrogen, and the percentage obtained with urea is the same.

• Do not mix serum urea with urine BUN, or the other way around.
• Both creatinines need to be in the same unit.
• Urea in mmol/L also works, as long as it is used in both samples.
• Zero, negative values or values above the limits produce an error message: check the value entered.
• The range is read from the displayed value: 35.004% appears as 35.00% and falls in the 35% or less range.

Checklist: simultaneous samples; same form of urea in both samples; creatinines in the same unit; record of diuretic use and urine output.

When FEUrea may be inappropriate

FEUrea has its own limitations, discussed in the literature since the original study. The main points of caution are:

• Variable performance: the 2024 meta-analysis found moderate sensitivity and specificity, well below those of the 2002 study.
• Population studied: Carvounis compared only prerenal azotemia and acute tubular necrosis. Other causes of acute kidney injury, such as obstruction or glomerulonephritis, were not part of the comparison.
• Intermediate range: between 35% and 50%, the result does not separate the two causes well; the 50% cutoff for an intrinsic cause comes from reviews, such as that by Gotfried and colleagues (2012), not from a dedicated validation study.
• Values close to the cutoffs: 34% and 36% have similar clinical meaning, even though they fall in different ranges.

FEUrea does not replace assessment of circulating volume, urine output and the course of creatinine. When urinary indices disagree, such as high FENa and low FEUrea in someone who has received a diuretic, the reading depends on the clinical context and is up to the attending physician.

FEUrea, FENa and other urinary indices

FEUrea and FENa use the same calculation structure and differ only in the solute. The table compares the two:

In the 2024 meta-analysis, in six studies of patients not on diuretics, FEUrea at 35% and FENa at 1% showed no significant difference in sensitivity (77% and 89%). Miller and colleagues, in 1978, also used the urine/serum urea ratio: above 8 suggested prerenal azotemia and below 3, acute tubular necrosis. Use the FENa calculator with the same samples to compare the two indices. The calculated serum osmolality also uses urea (or BUN) and helps read the fluid and electrolyte picture, and the spot urine protein complements the urine assessment.

Frequently asked questions about the fractional excretion of urea (FEUrea)

References

1. 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;62(6):2223-9. doi:10.1046/j.1523-1755.2002.00683.x. View
2. Abdelhafez MO, Alhroob AA, Abu Hawilla MO, Rjoob AA, Abualia NM, Gorman EF, et al. Utility of fractional excretion of urea in acute kidney injury with comparison to fractional excretion of sodium: a systematic review and meta-analysis. Am J Med Sci. 2024;368(3):224-34. doi:10.1016/j.amjms.2024.04.021. View
3. Miller TR, Anderson RJ, Linas SL, Henrich WL, Berns AS, Gabow PA, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50. doi:10.7326/0003-4819-89-1-47. View
4. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-81. doi:10.1001/jama.236.6.579. View
5. Gotfried J, Wiesen J, Raina R, Nally JV Jr. Finding the cause of acute kidney injury: which index of fractional excretion is better? Cleve Clin J Med. 2012;79(2):121-6. doi:10.3949/ccjm.79a.11030. View