FEUrea Calculator
Work out fractional excretion of urea from serum and urine labs, a perfusion marker that still works on diuretics.
What it does
Fractional excretion of urea, or FEUrea, estimates how much of the urea filtered by the kidney ends up in the urine. It is used when the more familiar fractional excretion of sodium may mislead, especially in people who have received diuretics. Loop and thiazide diuretics force sodium into the urine, which can make FENa look high even when the kidney is under perfused.
The calculation compares the urine to serum ratio of urea with the urine to serum ratio of creatinine. Creatinine is used as the marker of how much water was filtered and reabsorbed, while urea is the substance being tracked. Because the two ratios are divided, the units cancel as long as each substance is reported in the same unit on both sides of the fraction.
Kaplan and Kohn studied 93 patients and found that a FEUrea of 35% or less was a sensitive index of reduced renal perfusion, including after furosemide. Later work in pre-eclampsia reported the same pattern: values fell below 35% before delivery and normalized afterwards. The 35% cutoff has stayed in clinical use because it is simple and because it survives diuretic exposure better than sodium.
The test is not perfect. Values between 35% and 50% are ambiguous, and intrinsic kidney disease, urine collection errors and severe osmotic diuresis can all change the result. A low FEUrea is consistent with prerenal physiology, but it does not tell you whether the cause is bleeding, dehydration, heart failure or a third space loss.
Use the number next to the volume status, the creatinine trend, the urine output and the medicines. A single value taken before treatment is more useful than a value drawn after fluids and diuretics have already changed the picture.
How to use
- Enter the serum creatinine in mg/dL.
- Enter the serum urea in mg/dL. If your laboratory reports BUN, use a BUN based calculation instead, or convert consistently.
- Enter the urine creatinine and urine urea from the same collection period.
- Read the FEUrea percent and the reading band.
- Compare the result with the urine output, the creatinine trend and the medicines given.
- Repeat after treatment only if the repeat will change a decision.
Formula
- Urine urea
- Urea concentration in the urine, in the same unit as serum urea.
- Serum creatinine
- Creatinine concentration in the blood, in the same unit as urine creatinine.
- Serum urea
- Urea concentration in the blood, in the same unit as urine urea.
- Urine creatinine
- Creatinine concentration in the urine, in the same unit as serum creatinine.
The units cancel because the formula is a ratio of ratios, so mg/dL and mmol/L both work if each substance uses one unit throughout. If one laboratory reports BUN and another reports urea, the numbers are not interchangeable: BUN is reported as nitrogen, and urea is about 2.14 times larger by mass. The 35% cutoff comes from studies that used urea, so keep the input type straight. FEUrea is not the same test as FENa and the two should not be substituted for one another without checking the units and the clinical question.
Worked example
Example: serum creatinine 1.0, serum urea 40, urine creatinine 100, urine urea 800
Defaults: serum creatinine 1.0 mg/dL, serum urea 40 mg/dL, urine creatinine 100 mg/dL and urine urea 800 mg/dL.
- Multiply urine urea by serum creatinine: 800 x 1.0 = 800.
- Multiply serum urea by urine creatinine: 40 x 100 = 4,000.
- Divide the first product by the second: 800 / 4,000 = 0.20.
- Multiply by 100 to make a percent: 0.20 x 100 = 20.0 %.
- A value of 20.0 % is below the 35% cutoff, so it fits a prerenal pattern.
- The tool shows 20.0 % and the reading Low, fits a prerenal pattern.
Reference table
| FEUrea | Reading | What it suggests |
|---|---|---|
| 35% or less | Low | Reduced renal perfusion is likely, even if the patient received a diuretic |
| 36% to 50% | Borderline | The value does not settle the question; use the volume status and the creatinine trend |
| Above 50% | Not low | A prerenal pattern is less likely, though intrinsic kidney disease can still be present |
Limits
- Not a stand alone diagnosis, and a low value does not identify the cause of poor perfusion.
- Ambiguous in the 36% to 50% range, where clinical assessment matters more than the number.
- Urine collection errors, very low urine output and osmotic diuresis can distort the ratio.
- Requires urea values, not BUN values, unless the calculation and the cutoff are adjusted consistently.
Common mistakes
- Mixing BUN and urea in the same calculation without converting one to the other.
- Using a serum and urine sample from different collection periods.
- Reading a value above 35% as proof that the kidney is fine, when intrinsic disease can also raise the ratio.
- Applying a pediatric cutoff to an adult, or the reverse, without checking the source.
- Ordering a repeat after treatment and then interpreting it as if it were the baseline value.
Try it yourself
Quick checks. Answers sit tucked under each question.
Serum creatinine 1.2, serum urea 50, urine creatinine 80 and urine urea 600. What is the FEUrea?
600 x 1.2 = 720 and 50 x 80 = 4,000. Then 720 / 4,000 = 0.18, so FEUrea is 18.0 %, which is low.
Serum creatinine 1.0, serum urea 40, urine creatinine 100 and urine urea 1,600. What is the FEUrea?
1,600 x 1.0 = 1,600 and 40 x 100 = 4,000. Then 1,600 / 4,000 = 0.40, so FEUrea is 40.0 %, which is borderline.
Serum creatinine 0.9, serum urea 30, urine creatinine 120 and urine urea 900. What is the FEUrea?
900 x 0.9 = 810 and 30 x 120 = 3,600. Then 810 / 3,600 = 0.225, so FEUrea is 22.5 %, which is low.
Questions
What is a normal FEUrea?
There is no single normal value. In the perfusion studies, 35% or less was used as the low cutoff, and values above 50% were considered not low. The 36% to 50% range is best read with the clinical picture.
Why use FEUrea instead of FENa?
Diuretics raise sodium excretion and can make FENa look high even when perfusion is low. Urea handling is less affected by those drugs, so FEUrea can still show a prerenal pattern.
Does FEUrea replace FENa?
No. They answer related but different questions, and the choice depends on the medicines, the urine output and the suspected diagnosis. Some clinicians use both.
Can FEUrea be low in intrinsic kidney disease?
Yes. A low value is not specific for prerenal disease, and intrinsic or postrenal problems can produce misleading ratios. The trend in creatinine and the urine sediment often say more.
What if my laboratory reports BUN instead of urea?
BUN and urea are not the same number. If you only have BUN, use a BUN based formula or convert carefully, and do not apply a urea based cutoff to a BUN based ratio without checking.
How soon should I repeat the test?
There is no fixed schedule. Repeat only when the result will change treatment, and use the same collection method so the two values can be compared.
Sources
- Kaplan and Kohn: fractional excretion of urea as a guide to renal dysfunction, PubMed
- Fractional excretion of urea in pre-eclampsia, PubMed