What FENa tells you
When kidney function drops suddenly, acute kidney injury (AKI), one of the first questions is whether the kidneys are simply underperfused (a prerenal cause such as dehydration, bleeding or heart failure) or damaged themselves (an intrinsic cause such as acute tubular necrosis). Healthy tubules starved of blood flow hold on to sodium avidly; damaged tubules cannot. The fractional excretion of sodium (FENa) captures this difference.
The formulas
FENa (%) = (urine Na × serum creatinine) ÷ (serum Na × urine creatinine) × 100
FEUrea (%) = (urine urea × serum creatinine) ÷ (serum urea × urine creatinine) × 100
Both creatinine values must be in the same unit, and so must both urea values. Samples are taken at about the same time, before diuretics if possible.
Worked example
| Test | Serum | Urine |
|---|---|---|
| Sodium | 135 mmol/L | 20 mmol/L |
| Creatinine | 2.0 mg/dL | 100 mg/dL |
| Urea nitrogen | 40 mg/dL | 300 mg/dL |
- FENa = (20 × 2.0) ÷ (135 × 100) × 100 = 0.30%
- FEUrea = (300 × 2.0) ÷ (40 × 100) × 100 = 15%
Both point to a prerenal cause, such as volume depletion.
Interpretation
| Result | Prerenal | Indeterminate | Intrinsic |
|---|---|---|---|
| FENa | below 1% | 1–2% | above 2% |
| FEUrea | 35% or less | 35–50% | 50% or more |
When FENa misleads
- Diuretics force sodium into the urine and raise FENa even in prerenal states. FEUrea is more reliable in that case.
- Low FENa despite intrinsic injury: contrast nephropathy, rhabdomyolysis, early urinary obstruction, glomerulonephritis and sepsis.
- High FENa without AKI: chronic kidney disease, salt-wasting conditions and high salt intake.
- FENa is most useful in oliguric AKI, with low urine output.
Putting it together
FENa is a clue, not a diagnosis. Clinicians combine it with the history, blood pressure and fluid status, the BUN-to-creatinine ratio, urine microscopy (muddy brown casts suggest tubular necrosis) and the response to fluids.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.