Calculate fractional excretion of urea (FEUrea) from serum and urine urea and creatinine to tell prerenal AKI from ATN, even when the patient is on diuretics.
Fractional Excretion of Urea (FEUrea) Formula
FEUrea is the percentage of filtered urea that is excreted in the urine. It is calculated from paired serum and urine measurements of urea and creatinine:
FEUrea = (UUrea * SCr) / (SUrea * UCr) * 100
Variables:
- UUrea is the urine urea concentration
- SCr is the serum (plasma) creatinine concentration
- SUrea is the serum urea concentration, reported either as urea or as blood urea nitrogen (BUN)
- UCr is the urine creatinine concentration
The result is a percentage. Urea appears in both the numerator and the denominator, and creatinine does the same, so the units cancel as long as you enter both urea values in one unit and both creatinine values in one unit. You do not need to convert BUN into urea for the ratio to work.
The comparison mode adds the fractional excretion of sodium, which uses a serum and urine sodium value with the same pair of creatinine values:
FENa = (UNa * SCr) / (SNa * UCr) * 100
Each input maps to one part of the formula. The serum and urine urea set the numerator ratio, the two creatinine values scale that ratio to the amount of urea filtered, and the diuretic question changes how the number is read rather than the math itself. In comparison mode the sodium fields produce a second index so you can see whether FENa and FEUrea agree or point in different directions.
Reading Your FEUrea Result
Fractional excretion of urea places acute kidney injury into a prerenal pattern, a gray zone, or an intrinsic renal pattern. Treat the cutoffs as a guide that you read together with the history, exam, and other labs.
| FEUrea | Likely pattern | What it suggests |
|---|---|---|
| Under 35% | Prerenal | Volume depletion, heart failure, or another cause of low effective blood flow; tubules are reabsorbing urea avidly |
| 35% to 50% | Indeterminate | Overlap zone; repeat the test and correlate with the clinical course |
| Over 50% | Intrinsic renal | Acute tubular necrosis or other intrinsic injury; damaged tubules cannot reclaim urea |
The main reason to choose FEUrea over the fractional excretion of sodium is that it holds up when FENa does not. This table compares the two indices, including how each performs in patients who are already taking diuretics.
| Feature | FEUrea | FENa |
|---|---|---|
| Prerenal cutoff | Under 35% | Under 1% |
| Intrinsic cutoff | Over 50% | Over 2% |
| Reported sensitivity for prerenal AKI | About 85% to 90% | About 70% to 90% |
| Stays reliable during diuretic use | Yes | No |
| Prerenal cases correctly flagged while on diuretics | About 89% | About 48% |
In the Carvounis study of acute kidney injury, 89 percent of prerenal patients who were taking diuretics had a FEUrea under 35 percent, while only 48 percent of those same patients had a FENa under 1 percent. Diuretics force sodium out in the urine and push FENa up, but urea handling in the proximal tubule is far less affected, so FEUrea keeps pointing to the real cause. That gap is the reason to switch to FEUrea once a loop or thiazide diuretic is on board.
Example Problems
Example 1: prerenal pattern in a patient on a diuretic.
A patient with heart failure takes furosemide and has a rising creatinine. Labs come back as serum urea (BUN) 60, urine urea 400, serum creatinine 2.5, and urine creatinine 75, with urea and creatinine each in mg/dL. Put the numbers into the formula:
FEUrea = (400 * 2.5) / (60 * 75) * 100 = 22.2 percent. That is under 35 percent, so the result fits a prerenal cause even though the diuretic would make FENa hard to trust.
Example 2: intrinsic pattern.
A different patient has serum urea 25, urine urea 150, serum creatinine 3, and urine creatinine 30. Run the same formula:
FEUrea = (150 * 3) / (25 * 30) * 100 = 60 percent. That is above 50 percent, which points to intrinsic injury such as acute tubular necrosis.
Frequently Asked Questions
Why use FEUrea instead of FENa?
FENa becomes unreliable when a patient is on diuretics, because the drug increases sodium excretion and drives FENa upward even when the kidney is volume depleted. Urea is reabsorbed mainly in the proximal tubule and is much less affected by diuretics, so FEUrea stays accurate in that setting. When someone is taking a loop or thiazide diuretic, FEUrea is the index to trust.
What is a low FEUrea?
A FEUrea under 35 percent is considered low and suggests a prerenal cause, where the kidney is holding on to urea in response to reduced blood flow. A value over 50 percent suggests intrinsic renal injury such as acute tubular necrosis, and 35 to 50 percent is a gray zone. Read the number together with urine output, the serum creatinine trend, and the clinical history rather than on its own.
What can make FEUrea misleading?
Anything that changes urea production or handling can shift the result. Gastrointestinal bleeding, a high protein or catabolic state, and corticosteroids raise urea and can pull FEUrea down toward a falsely prerenal reading. Low protein intake, the syndrome of inappropriate antidiuretic hormone, and older age can raise it. The test also needs some urine flow to be meaningful, so it is not useful in complete anuria.