Introduction
The FEUrea calculator works out the fractional excretion of urea. This is the percent of urea your kidneys filter out and then send into your urine instead of taking it back into your blood. It helps show why a person's kidney numbers are off.
When someone has sudden kidney trouble (acute kidney injury), doctors want to know the cause. Is the problem low blood flow to the kidneys, called pre-renal? Or is the kidney tissue itself hurt, called intrinsic renal damage, such as ATN? FEUrea helps tell these apart. It is often used instead of FENa because it still works when a patient is taking a diuretic (water pill).2
To use this tool, you need four lab values:
- Serum urea or BUN (from a blood test)
- Serum creatinine (from a blood test)
- Urine urea (from a urine sample)
- Urine creatinine (from a urine sample)
You can enter each one in mg/dL, mmol/L, or µmol/L. The calculator changes them all to the same unit for you.
Your result lands in one of three zones. Under 35% points to pre-renal causes, like dehydration.1 Between 35% and 50% is unclear and needs more testing. Over 50% points to damage inside the kidney.1 The tool also shows each math step and a chart, so you can see how the number was found.
This calculator gives information only. It does not replace a doctor. Always rule out a blocked urinary tract before you use this result to guide care.
How to use our FEUrea Calculator
Enter four lab values from a blood test and a urine test. The calculator gives you the fractional excretion of urea (FEUrea) as a percent, tells you if the result points to a pre-renal cause, an intrinsic kidney cause, or an unclear middle zone, and shows the math step by step.
Serum Urea / BUN (SUrea): Type the blood urea nitrogen value from the patient's BMP or CMP. Pick mg/dL or mmol/L in the drop-down next to the box. A normal blood level is generally 6 to 20 mg/dL.3
Serum Creatinine (SCr): Type the creatinine value from the same blood panel. Pick mg/dL or µmol/L. A normal blood level is 0.7 to 1.3 mg/dL for men and 0.5 to 0.95 mg/dL for women.4
Urine Urea (UUrea): Type the urea value from the urine sample. Pick mg/dL or mmol/L. Urine urea is much higher than blood urea, often 1,200 to 2,000 mg/dL.
Urine Creatinine (UCr): Type the creatinine value from the same urine sample. Pick mg/dL or µmol/L. Urine creatinine often falls between 20 and 320 mg/dL.
Click Calculate to see your FEUrea result, the color zone it lands in, and the full solution. Click Reset to clear every box and start again. If you switch a unit, the number you typed is converted for you.
What Is the Fractional Excretion of Urea (FEUrea)?
The fractional excretion of urea, or FEUrea, is a simple lab test result that shows how much of the urea filtered by your kidneys ends up in your urine. Urea is a waste product your body makes when it breaks down protein. Healthy kidneys filter urea out of the blood, then take some of it back. FEUrea tells doctors how much the kidneys are holding on to.
Why Doctors Use It
When someone has sudden kidney trouble, called acute kidney injury (AKI), doctors need to find the cause. FEUrea helps sort two main groups:
- Pre-renal AKI. The kidneys are fine, but not enough blood is reaching them. This can happen with dehydration, blood loss, heart failure, or liver disease. The kidneys hold on to water and urea, so FEUrea is low.1
- Intrinsic renal AKI. The kidney tissue itself is damaged, often from acute tubular necrosis (ATN), low blood flow over time, or drugs that harm the kidney. Damaged tubules cannot hold on to urea, so FEUrea is high.
What the Numbers Mean
- Under 35%. A pre-renal cause is more likely.1 Fluids often help.
- 35% to 50%. Unclear. More tests and a closer look at the patient are needed.
- Over 50%. Intrinsic kidney damage, such as ATN, is more likely.1
FEUrea vs. FENa
Doctors usually learn the fractional excretion of sodium (FENa) first. The problem is that water pills (diuretics like furosemide) push sodium into the urine and make FENa look high, even when the real problem is low blood flow.2 Urea is handled in other parts of the kidney, so FEUrea stays useful in patients taking diuretics.1 That is the main reason it is ordered.
What You Need for the Test
You need four lab values, drawn close together in time:
- Serum urea or BUN (blood urea nitrogen) from a blood test
- Serum creatinine from the same blood test
- Urine urea from a spot urine sample
- Urine creatinine from that same urine sample
The formula is: FEUrea = (Serum Creatinine × Urine Urea) ÷ (Serum Urea × Urine Creatinine) × 100.1 All four values must be in the same unit before you divide.
Things That Can Throw It Off
FEUrea is a clue, not a final answer. It can be misleading in older adults, whose kidneys hold on to less sodium and urea, and during an infection such as sepsis.1 High-protein feeds and gut bleeding raise urea and can also throw it off.3 So can chronic kidney disease and steroid medicines. Blocked urine flow (obstruction) should be ruled out first, usually with an ultrasound. Always read the number next to the patient's history, exam, urine test, and other labs.