Hong Life CalculatorsUnderstanding the FEUrea calculator — concept and principle
The fractional excretion of urea (FEUrea) is the share of urea filtered by the kidneys that ends up in the urine. Clinicians use it in acute kidney injury to help tell a prerenal cause (low blood flow to the kidneys) from intrinsic kidney damage such as acute tubular necrosis, especially in patients who have received diuretics.
How it is calculated
FEUrea (%) = (urine urea × serum creatinine) ÷ (serum urea × urine creatinine) × 100
- Urea can be entered as urea or as blood urea nitrogen (BUN), as long as serum and urine use the same unit.
- Creatinine in serum and urine must also share a unit. The units cancel out.
- The calculator also shows the urine/plasma ratios for urea and creatinine.
The principle
Creatinine is filtered and hardly reabsorbed, so its urine/plasma ratio indicates how much the urine has been concentrated. Comparing urea's ratio with creatinine's shows what share of filtered urea escaped reabsorption. When the kidneys are under-perfused, they save water and urea along with it, so FEUrea falls, usually below 35%. When the tubules are injured they cannot reabsorb well, and FEUrea is usually above 50%. Urea is reabsorbed mainly in the proximal tubule, so loop and thiazide diuretics, which strongly raise the fractional excretion of sodium (FENa), affect it much less.
Worked example
Default inputs: serum urea 40, urine urea 600, serum creatinine 1.5, urine creatinine 120.
- Numerator = 600 × 1.5 = 900.
- Denominator = 40 × 120 = 4,800.
- FEUrea = 900 ÷ 4,800 × 100 = 18.8% → Under 35% – prerenal pattern.
- Urine/plasma urea = 600 ÷ 40 = 15; urine/plasma creatinine = 120 ÷ 1.5 = 80.
| Serum (plasma) urea or BUN | Fractional excretion of urea (FEUrea) |
|---|---|
| 10 | 75% |
| 20 | 37.5% |
| 30 | 25% |
| 40 | 18.8% |
| 50 | 15% |
| 60 | 12.5% |
| 80 | 9.4% |
How to read the result
Under 35% supports a prerenal state such as dehydration, bleeding or heart failure. Over 50% supports intrinsic renal injury. Values from 35% to 50% are indeterminate. The test helps rather than decides: it is less reliable in sepsis, in older adults and in chronic kidney disease.
Common mistakes
- Mixing units between the serum and urine values of the same substance.
- Using samples collected at very different times.
- Relying on FEUrea alone without history, examination and urine microscopy.
Frequently asked questions
When is FEUrea better than FENa?
When the patient has had diuretics, which raise sodium excretion and can make FENa misleading.
Can I enter BUN instead of urea?
Yes, if both the serum and urine values are BUN or urea nitrogen.
What does an indeterminate result mean?
A value of 35–50% does not clearly point either way, so other findings carry more weight.
Sources: Carvounis CP, Nisar S, Guro-Razuman S, "Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure", Kidney Int 2002;62:2223–2229