FEUrea Calculator (Fractional Excretion of Urea) | Diuretic AKI Tool

Why FEUrea beats FENa

Loop diuretics (like Furosemide/Lasix) work by blocking sodium reabsorption in the Loop of Henle. This artificially doubles or triples the amount of sodium in the urine, making FENa tests uselessly high (> 2%).

However, urea reabsorption primarily happens in the Proximal Tubule, which diuretics generally do not affect. This makes FEUrea a "clean" window into the kidney's intent—is it trying to save water (Prerenal) or is it too damaged to function (Intrinsic)?

Interpreting the Result

< 35% (Prerenal)

The kidney is functioning correctly. It is sensing low volume (dehydration) and actively reabsorbing urea along with water. The treatment is typically IV fluids.

> 50% (Intrinsic / ATN)

The proximal tubules are damaged or necrotic. They cannot reabsorb urea even if the body is dehydrated. This indicates ATN regardless of diuretic use.

Frequently Asked Questions

Is FEUrea more accurate than FENa?

Only in specific contexts. For patients not on diuretics, FENa is still the gold standard because sodium handling is the kidney's primary function. FEUrea is specifically superior for patients taking diuretics (Lasix/Bumex) because urea transport is unaffected by these drugs, avoiding the "false positive" ATN diagnosis that FENa often triggers.

Can I use FEUrea for patients with sepsis?

Yes, but with caution. Sepsis causes complex hemodynamic changes. While FEUrea is useful, sepsis often causes a "mixed" picture of both pre-renal low flow and toxic damage to the tubules. Most nephrologists look for a trend (e.g., is FEUrea rising or falling?) rather than a single snapshot in septic patients.

What if the result is between 35% and 50%?

This "Gray Zone" is diagnostic purgatory. It suggests a transition phase where dehydration (Prerenal) has lasted long enough to start causing actual tissue damage (ATN). In these cases, doctors rely on other signs like urine microscopy (checking for muddy brown casts) or response to a fluid challenge to make the final call.

Does a high BUN affect the calculation?

Yes, mathematically, but that's the point. BUN (Blood Urea Nitrogen) rises when kidneys fail. The formula compares the BUN to the Urine Urea. If your BUN is high but your Urine Urea is also high, the fraction remains normal. It is the ratio that matters, not the raw number.

Why is the cutoff 35% and not 1% like FENa?

Because the kidney handles urea differently than sodium. Sodium is precious, so the kidney reabsorbs 99% of it (FENa < 1%). Urea is a waste product, so the kidney naturally excretes more of it. Even when "saving water," the kidney still dumps about 30-35% of the urea, hence the higher cutoff threshold.

References

Carvounis CP, et al. (2002). Significance of fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney International.
Medscape. Fractional Excretion of Urea Analysis.
Renal Fellow Network. FENa vs FEUrea: The Debate.
National Institutes of Health (NIH). Urea Nitrogen Handling in AKI.

Tools & Data Verified by the EverydayCalculators Medical Research Team.

Last updated: January 2026.

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