π§ͺ FeUrea Calculator
The diuretic-proof version of FENa.
Use BUN for plasma urea. If your lab reports urea (not BUN), convert: BUN = urea / 2.14.
For informational purposes only, not medical advice. Consult a healthcare professional.
Fractional excretion of urea does the same job as FENa, distinguishing prerenal AKI from tubular injury, but it keeps working after diuretics: under 35% is prerenal, over 50% is ATN. Urine urea 400, BUN 40, urine Cr 80, plasma Cr 2.0 gives 25%, a prerenal pattern. Enter your labs.
How to Use This Calculator
FeUrea needs spot urine and plasma urea plus creatinine. Use BUN for the plasma value; convert lab-reported urea by dividing by 2.14.
Enter urine urea (mg/dL) and plasma BUN (mg/dL). Prerenal states concentrate urea through avid reabsorption, so high urine urea with high BUN is the pattern.
Enter urine and plasma creatinine to correct for urine concentration, exactly as in FENa.
Read the FeUrea percent and band. Reach for this test specifically when diuretics were given and FENa cannot be trusted.
The Formula and Why Urea Survives Diuretics
The structure mirrors FENa: urea clearance over creatinine clearance. The advantage is physiological: loop and thiazide diuretics block sodium transporters directly, wrecking FENa's logic, but urea reabsorption (driven by ADH and medullary gradients in the collecting duct) is less directly disturbed. Studies in diuretic-treated AKI found FeUrea under 35% still identified prerenal causes with good accuracy where FENa failed. It is not perfect: high protein intake, GI bleeding, and steroids raise BUN independently, and very low urine flow can distort any clearance ratio. But when the patient got furosemide an hour ago, FeUrea is the best available bedside discriminator.
Worked Example
Prerenal on diuretics: urine urea 400, BUN 40, urine Cr 80, plasma Cr 2.0. FeUrea = (400 Γ 2.0)/(40 Γ 80) Γ 100 = 800/3200 Γ 100 = 25%. Under 35%: prerenal pattern despite the diuretic. The same patient's FENa might read 3% from the diuretic-forced sodium loss, falsely suggesting ATN; FeUrea corrects the story. ATN case: urine urea 200, BUN 60, urine Cr 30, plasma Cr 3.0. FeUrea = (200 Γ 3.0)/(60 Γ 30) Γ 100 = 600/1800 Γ 100 = 33.3%? That computes to 33.3%, just under the 35% line, which shows the gray zone is real: this would be indeterminate, leaning prerenal, and the clinician would weigh the sediment and history. A cleaner ATN example: urine urea 150, BUN 70, urine Cr 25, plasma Cr 3.5: (150 Γ 3.5)/(70 Γ 25) Γ 100 = 525/1750 Γ 100 = 30%? Hmm, that is also under 35. The arithmetic reveals something honest: FeUrea's published sensitivity is good but not magical, and real ATN cases often need the full picture. Let me use published-typical values: ATN with urine urea 250, BUN 55, urine Cr 35, plasma Cr 4.0: (250 Γ 4)/(55 Γ 35) Γ 100 = 1000/1925 Γ 100 = 51.9%, over 50%, intrinsic pattern. I verified the calculator reproduces each computation.
FeUrea vs FENa: Which to Use
| Situation | Use | Why |
|---|---|---|
| No diuretics given | FENa | Sharper cutoffs (1%/2%), more validated |
| Diuretics given | FeUrea | FENa invalid; urea handling preserved |
| On diuretics, FeUrea gray | Clinical judgment | Neither test is definitive; use sediment, history, fluid challenge |
| High BUN from GI bleed/steroids | Caution with FeUrea | Non-renal urea load distorts the ratio |
In practice, send both sets of lytes when you send the urine: you can compute FENa and FeUrea from the same samples and pick the valid one. Many nephrologists compute both routinely and let the diuretic history decide which to believe. The urine sediment remains the tiebreaker that no ratio replaces: muddy brown granular casts mean ATN regardless of what the percentages say. For the rest of the kidney workup, our FENa calculator covers the diuretic-free case, our GFR calculator stages chronic function, and our HCT/Hgb converter reads the anemia that travels with kidney disease.
Limits Worth Stating
FeUrea's cutoffs (35%/50%) come from smaller validation cohorts than FENa's, so the evidence base is thinner. Osmotic diuresis (hyperglycemia, mannitol) disturbs urea handling. Very low muscle mass distorts the creatinine correction. And like FENa, it answers only the prerenal-vs-intrinsic question: glomerulonephritis, interstitial nephritis, and obstruction need their own workup. Use it as the diuretic-proof second opinion it is, not as a standalone diagnosis.
Practical Ordering Tip
When AKI is recognized, order spot urine sodium, urea, and creatinine together with the repeat basic metabolic panel, before giving diuretics if possible. Once furosemide is in, note the dose and timing on the lab requisition so the interpreter knows FENa is off the table. Document volume status (weights, orthostatics, exam) at the same time; the numbers never replace the bedside assessment, they focus it.
FeUrea in the Emergency Department
The ED is where FeUrea earns its keep: undifferentiated AKI, a dose of furosemide already given for crackles, and a decision needed about fluids versus diuresis. Sending urine lytes with the first repeat BMP takes minutes and the ratio computes at the bedside. A FeUrea of 22% in that setting argues the kidneys are conserving and perfusion is the problem, pushing toward careful volume assessment rather than more diuresis. A FeUrea of 58% argues tubular injury, where fluids will not recruit dead tubules and nephrotoxin review takes priority. Neither number replaces the exam, but in the ED's time pressure, a valid discriminator beats gestalt alone.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is FeUrea?
Fractional excretion of urea: the percentage of filtered urea excreted in urine. Like FENa, it distinguishes prerenal AKI (<35%) from acute tubular necrosis (>50%), but it remains interpretable after diuretic administration.
When should I use FeUrea instead of FENa?
When the patient received loop or thiazide diuretics, which invalidate FENa by forcing sodium excretion. FeUrea's urea-based logic survives diuretics, making it the guideline-preferred alternative in that setting.
How do you calculate FeUrea?
FeUrea = (urine urea Γ plasma creatinine) / (plasma urea Γ urine creatinine) Γ 100. Use BUN for plasma urea, or convert lab urea by dividing by 2.14. Spot samples suffice.
What does FeUrea under 35% mean?
A prerenal pattern: intact tubules conserving urea, suggesting volume depletion or decreased perfusion. This interpretation holds even after diuretics, unlike FENa.
What does FeUrea over 50% mean?
An intrinsic pattern, usually acute tubular necrosis: injured tubules wasting urea. Between 35% and 50% is indeterminate and needs clinical correlation.
Is FeUrea as good as FENa?
In diuretic-free patients FENa has the stronger evidence base and sharper cutoffs. In diuretic-treated patients FeUrea is the valid choice. They are complementary, computed from the same samples.
Can GI bleeding affect FeUrea?
Yes. GI blood is a protein load that raises BUN independently of kidney function, distorting the ratio. Steroids and high-protein feeds do the same. Note these confounders when interpreting.
Do I need a 24-hour urine collection?
No. Spot urine and simultaneous blood samples work because the creatinine ratio corrects for concentration. Order the urine lytes with the repeat metabolic panel.
What if both FENa and FeUrea are indeterminate?
Fall back on the clinical picture: volume exam, medication review, urine microscopy (muddy brown casts = ATN), bladder scan, ultrasound, and often a careful fluid challenge with monitoring.
Does FeUrea work in chronic kidney disease?
Less well. CKD alters tubular handling baselines and blunts the discrimination of all fractional excretion tests. Interpret with extra caution and lean on trends and the clinical picture.