Calculate fractional excretion of urea (FEUrea) from serum and urine urea or BUN and creatinine, with an optional FENa comparison from sodium values.
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. Each blood/urine pair must use matching units and analyte basis, or be converted. BUN and urine urea nitrogen share a nitrogen basis; whole urea mg/dL is not interchangeable with urea nitrogen mg/dL. Select each reported basis and unit in this calculator.
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
Conventional FEUrea ranges can support a prerenal or tubular-dysfunction interpretation, but overlap is substantial. Read them alongside the history, examination, urine output and other laboratory findings; they are not diagnostic categories.
| FEUrea | Likely pattern | What it suggests |
|---|---|---|
| Under 35% | May support prerenal physiology | 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% | May support tubular dysfunction | May be seen with tubular dysfunction, but is not specific for its cause |
FEUrea may add context when sodium handling is altered, but neither index is universally reliable. The following are conventional ranges, not guarantees of diagnostic accuracy.
| Feature | FEUrea | FENa |
|---|---|---|
| Prerenal cutoff | Under 35% | Under 1% |
| Intrinsic cutoff | Over 50% | Over 2% |
| Diagnostic performance | Varies by population | Varies by population |
| Diuretic limitation | Can be affected | Can be affected |
| Use in clinical decisions | One contextual index | One contextual index |
In the original Carvounis cohort, a FEUrea cutoff of 35% performed well in selected prerenal cases, including diuretic users. Later studies found lower discrimination in intensive care and sepsis. A prospective heart-failure study also found that loop diuretics changed FEUrea and moved 27% of patients across the 35% threshold. See Carvounis et al., Darmon et al. and Cox et al..
Example Problems
Example 1: prerenal pattern in a patient on a diuretic.
Example inputs from paired specimens: serum BUN 60 mg/dL, urine urea nitrogen 400 mg/dL, serum creatinine 2.5 mg/dL and urine creatinine 75 mg/dL. Both urea measurements use the nitrogen basis:
FEUrea = (400 * 2.5) / (60 * 75) * 100 ≈ 22.2 percent. This is below the conventional 35% threshold and may support a prerenal pattern, but does not establish a diagnosis or remove diuretic-related uncertainty.
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. This is above the conventional 50% threshold and may support tubular dysfunction; clinical correlation is required.
Frequently Asked Questions
Why use FEUrea instead of FENa?
Diuretics increase sodium excretion and can limit FENa interpretation. FEUrea may provide additional context, but is also affected by diuretics and is not a reliable standalone substitute in every patient.
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?
Specimen timing, mismatched units or analyte basis, altered urea production and handling, diuretics, and sepsis can complicate interpretation. Low FEUrea may occur with intrinsic injury. These measurements require urine and must be interpreted by a qualified clinician.
