Why use FEUrea instead of FENa?

Loop and thiazide diuretics force sodium excretion, which falsely elevates FENa and makes it unreliable in patients receiving these drugs. Urea reabsorption is largely passive and not significantly affected by most diuretics, so FEUrea retains its discriminating value even in patients on active diuretic therapy.

What FEUrea value indicates prerenal AKI?

A FEUrea below 35% suggests the tubules are intact and reabsorbing urea avidly, consistent with prerenal causes such as volume depletion, cardiorenal syndrome, or hepatorenal syndrome. The kidney is responding appropriately to reduced effective perfusion.

What FEUrea value indicates intrinsic renal AKI?

A FEUrea above 50% suggests tubular injury — most often acute tubular necrosis (ATN) — where damaged tubules can no longer reabsorb urea efficiently. Values between 35% and 50% are indeterminate and require clinical correlation with history, urinalysis, and microscopy.

What units should I enter?

Urine urea and serum urea (or BUN) must use the same units — both in mg/dL (BUN) or both in mmol/L (urea). Urine creatinine and serum creatinine must also match — both mg/dL or both µmol/L. The formula is a ratio, so matching units cancel correctly. Mixing units gives an incorrect result.

Are there conditions that give a falsely low FEUrea?

Yes. High-protein intake, gastrointestinal bleeding, and glucocorticoids can elevate BUN independently of renal function, potentially lowering FEUrea even in intrinsic AKI. Conversely, low protein intake lowers BUN and may raise FEUrea despite prerenal disease. Always interpret in the full clinical context.

Can I use FEUrea and FENa together?

Yes. Using both values provides complementary information. When FENa is unreliable (patient on diuretics), FEUrea becomes the primary test. When FEUrea is borderline (35–50%), a reliable FENa or urine microscopy findings (muddy brown casts = ATN) can tip the diagnosis. Neither test replaces clinical judgment.

Fractional Excretion of Urea Calculator

Name: Fractional Excretion of Urea Calculator
Availability: InStock
Author: Nham Vu

Calculate FEUrea (%) from serum and urine urea and creatinine to differentiate prerenal from intrinsic AKI, especially when the patient is on diuretics.

Enter Lab Values

Use a spot urine and serum sample drawn at the same time. Urea and creatinine pairs must share the same units.

Urine Urea / BUN (U_Urea) mg/dL or mmol/L

Serum Urea / BUN (S_Urea) same units as urine urea

Urine Creatinine (U_Cr) mg/dL or µmol/L

Serum Creatinine (S_Cr) same units as urine creatinine

Formula

FEUrea (%) = (U_Urea × S_Cr) / (S_Urea × U_Cr) × 100

Clinical Note

FEUrea is preferred over FENa when the patient is on loop or thiazide diuretics. Neither test replaces clinical judgment — always correlate with history, urine microscopy, and imaging. FEUrea can be unreliable in high-protein states or GI bleeding that independently elevates BUN.

Summary

Calculate FEUrea (%) from serum and urine urea and creatinine to differentiate prerenal from intrinsic AKI, especially when the patient is on diuretics.

How it works

Collect a spot urine sample and a blood (serum) sample at the same time.
Enter the urine urea nitrogen (BUN) in mg/dL, or urine urea in mmol/L — use the same units as the serum value.
Enter the serum BUN (or serum urea) in the same units.
Enter urine creatinine and serum creatinine in the same units (both mg/dL or both µmol/L).
The calculator applies: FEUrea (%) = (U_Urea × S_Cr) / (S_Urea × U_Cr) × 100.
Review the result: <35% suggests prerenal AKI; >50% suggests intrinsic renal injury; 35–50% is indeterminate.

Use cases

Evaluating AKI in patients already receiving loop or thiazide diuretics, where FENa is unreliable.
Differentiating prerenal azotemia from acute tubular necrosis in the ICU.
Guiding fluid resuscitation decisions in oliguric patients on chronic diuretic therapy.
Supplementing FENa interpretation when the result falls in the indeterminate 1–2% range.
Nephrology consult triage when the cause of rising creatinine is unclear.
Medical education for teaching renal physiology and AKI diagnostic frameworks.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu