Last updated October 5, 2026 · Report an error
Fractional Excretion of Sodium (FENa), the standard published formula used to help distinguish prerenal from intrinsic causes of acute kidney injury: FENa % = (Urine Na x Serum Creatinine) / (Serum Na x Urine Creatinine) x 100. Pure arithmetic on four lab values you provide. Commonly cited interpretation bands: under 1% is often discussed as suggestive of a prerenal cause, over 2% as suggestive of intrinsic renal causes (such as acute tubular necrosis) — reference only, not an interpretation of any specific patient. For general reference and educational purposes only — not a medical diagnosis. Consult a doctor for actual kidney function assessment.
How it works
Enter urine sodium, serum sodium, urine creatinine, and serum creatinine. The tool applies the standard FENa formula.
- Enter urine sodium (mEq/L).
- Enter serum sodium (mEq/L).
- Enter urine creatinine (mg/dL).
- Enter serum creatinine (mg/dL).
- Click Calculate to see your results.
Examples
Urine Na 20, Serum Na 140, Urine Cr 60, Serum Cr 1.5
FENa = (20 x 1.5) / (140 x 60) x 100 = 30 / 8400 x 100 ≈ 0.36% — commonly cited as suggestive of a prerenal cause.
Who should use it
- Reviewing acute kidney injury labs alongside a doctor.
- Nephrology and critical care education about AKI classification.
Industry applications
- Nephrology and critical care reference
- Emergency medicine education
Advantages
- Standard, widely taught formula.
- Uses four routinely available lab values.
Limitations
- Less reliable in patients on diuretics.
- A single-point calculation — not a complete clinical picture on its own.
Common mistakes to avoid
- Using FENa in a patient on diuretics, where it is commonly considered less reliable (FEUrea is preferred in that setting).
- Mixing up which values are "urine" versus "serum" when entering the four labs.
Best practices
- Consider FEUrea instead of FENa in patients on diuretics, per commonly cited nephrology guidance.
Tips
- FENa is typically interpreted alongside urine output, urine sediment, and the overall clinical picture, not in isolation.