For healthcare professionals
FEUrea Calculator (Fractional Excretion of Urea)
The fractional excretion of urea (FEUrea) is the share of filtered urea that ends up in the urine. In acute kidney injury, a value under 35% traditionally points to a transient (prerenal) cause and a value over 35% to a persistent (intrinsic) cause. It is often used instead of FENa in patients on diuretics. This tool is written for clinicians and is not a diagnosis.
Laboratory values
Use a spot urine sample and a blood sample taken at about the same time. Nothing you enter leaves this page.
What you will see
The FEUrea as a percentage, the working, and how the value reads against the 35% cut point.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formula and the 35% reference value as printed in the review by Lima C and Macedo E, Disease Markers 2018 (Table 1, modified from Carvounis 2002), and the formula as defined by Darmon M and colleagues, Critical Care 2011. Both are open access (CC BY). The tests check the formula, the 35% cut point, that the units cancel out and that a value near 35% is never shown as 35.0.
Validated in
Adults with acute kidney injury. Darmon 2011 studied 203 adults admitted to three university hospital intensive care units, excluding obstructive kidney injury. Vanmassenhove 2013 studied adults with sepsis in one intensive care unit in Belgium.
Limits
FEUrea is a ratio, not a validated risk score. In Darmon 2011 it separated transient from persistent acute kidney injury poorly (area under the curve 0.59; sensitivity 63% and specificity 54% at 35%), with similar results in patients on diuretics. In early sepsis, most patients had a FEUrea under 35% (Vanmassenhove 2013), so a low value does not rule out kidney injury there.
The formula
FEUrea (%) = (urine urea / plasma urea) / (urine creatinine / plasma creatinine) x 100 (Lima 2018) (Darmon 2011). This is the same as (urine urea x plasma creatinine) / (plasma urea x urine creatinine) x 100. Each pair must be in one unit; the units then cancel out.
Worked example: urine urea 300 mg/dL, plasma creatinine 3.0 mg/dL, plasma urea 60 mg/dL and urine creatinine 100 mg/dL give (300 x 3.0) / (60 x 100) x 100 = 15%, under 35%.
Reading the result
- Under 35%: the traditional pattern of transient (prerenal) acute kidney injury (Lima 2018).
- Over 35%: the traditional pattern of persistent (intrinsic) acute kidney injury (Lima 2018).
The tool bands the unrounded value; exactly 35% sits on the cut point and is reported as such.
Why FEUrea and what the studies found
Diuretics and sepsis interfere with FENa. Urea is reabsorbed mainly in the proximal tubule and is not affected by diuretics, so FEUrea may be more reliable than FENa in these patients (Darmon 2011). Lima and Macedo describe a prospective study by Carvounis and colleagues in which a FEUrea under 35% was associated with a 98% chance of prerenal failure, with a sensitivity of 85% and a specificity of 92% (Lima 2018).
Later work was less favorable. In 203 critically ill adults, the median FEUrea was 41% in transient and 32% in persistent acute kidney injury; at the 35% cut point it predicted persistent injury with 63% sensitivity and 54% specificity, also in patients on diuretics, and the authors concluded it may be of little help in the intensive care unit (Darmon 2011). In adults with sepsis, 63.2% had a FEUrea under 35% at admission, and the authors suggest lower cut-off values may be needed in sepsis (Vanmassenhove 2013).
Related tools
The FENa calculator uses sodium in place of urea. The BUN to creatinine ratio calculator needs only blood values. To estimate kidney function, see the creatinine clearance calculator and the eGFR calculator. Other bedside scores are listed under For healthcare professionals.
Sources
- Lima C, Macedo E. Urinary biochemistry in the diagnosis of acute kidney injury. Dis Markers. 2018;2018:4907024. doi:10.1155/2018/4907024. PMID 30008975. Open access (CC BY). Table 1 (formula; under 1% transient, over 1% persistent AKI) and text (Espinel; glomerulonephritis; obstruction; fractional excretion of urea). pmc.ncbi.nlm.nih.gov/articles/PMC6020498/
- Darmon M, Vincent F, Dellamonica J, Schortgen F, Gonzalez F, Das V, Zeni F, et al. Diagnostic performance of fractional excretion of urea in the evaluation of critically ill patients with acute kidney injury: a multicenter cohort study. Crit Care. 2011;15(4):R178. doi:10.1186/cc10327. PMID 21794161. Open access (CC BY 2.0). Table 1 note (FEUrea formula), Results and Table 2 (performance at the 35% cut-off). pmc.ncbi.nlm.nih.gov/articles/PMC3387621/
- Vanmassenhove J, Glorieux G, Hoste E, Dhondt A, Vanholder R, Van Biesen W. Urinary output and fractional excretion of sodium and urea as indicators of transient versus intrinsic acute kidney injury during early sepsis. Crit Care. 2013;17(5):R234. doi:10.1186/cc13057. PMID 24119730. Open access (CC BY). Methods (formula; 107 consecutive sepsis patients) and Discussion (75% under 1%; diuretics). pmc.ncbi.nlm.nih.gov/articles/PMC4057160/
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