FENa and FEUrea calculator
Enter sodium (or urea) and creatinine measured in plasma and in a urine sample taken at the same time to get the fractional excretion and its usual reading in acute kidney injury. Information for clinicians, not a diagnosis.
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How to use it
- Choose FENa or FEUrea.
- Enter the plasma and urine sodium (or urea) from samples taken at about the same time.
- Enter plasma and urine creatinine in the same unit, mg/dL or µmol/L.
- Read the fractional excretion and its usual reading.
How it works
FE = (urine X × plasma creatinine) ÷ (plasma X × urine creatinine) × 100, where X is sodium or urea.
| Result |
FENa |
FEUrea |
| Prerenal |
below 1% |
35% or less |
| Indeterminate |
1–2% |
35–50% |
| Intrinsic |
above 2% |
above 50% |
Worked example
Plasma sodium 140 mmol/L, urine sodium 20 mmol/L, plasma creatinine 2 mg/dL, urine creatinine 100 mg/dL. FENa = (20 × 2) ÷ (140 × 100) × 100 = 0.29%, prerenal.
The same patient after furosemide, with plasma urea 20 and urine urea 300 (same unit): FEUrea = (300 × 2) ÷ (20 × 100) × 100 = 30%, still prerenal.
Common mistakes
- Mixing units: plasma creatinine in µmol/L with urine creatinine in mg/dL gives a wrong result.
- Using samples taken hours apart.
- Applying the cut-offs to a patient who is passing normal amounts of urine; they were described for oliguric kidney injury.
- The result is information and does not replace clinical judgment.
Frequently asked questions
What is the FENa formula?
FENa = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. The two creatinine values must be in the same unit, but which unit does not matter, because it cancels out.
How is FENa interpreted?
In oliguric acute kidney injury, below 1% suggests a prerenal cause such as dehydration or low blood flow, and above 2% suggests damage to the kidney itself, such as acute tubular necrosis. Values from 1 to 2% are indeterminate.
When is FEUrea better than FENa?
After diuretics, which make the kidney lose sodium and push FENa up even in prerenal states. Urea handling is less affected, so FEUrea of 35% or less suggests prerenal and above 50% intrinsic injury.
Are there exceptions?
Yes. FENa can be low in some intrinsic causes, such as contrast nephropathy, rhabdomyolysis or glomerulonephritis, and high in chronic kidney disease. The result is one clue among several.