FENa Calculator (Fractional Excretion of Sodium)
Differentiate prerenal from intrinsic renal azotemia using urine and serum sodium and creatinine.
Understanding this tool
Why nurses use it
Fractional excretion of sodium (FENa) helps differentiate prerenal azotaemia from intrinsic renal injury in AKI workups — when interpreted with diuretics and clinical context.
When to use it
Use in oliguric AKI when cause is unclear — invalid if patient received diuretics (consider FEUrea instead).
Calculate
How to use this calculator
- Enter urine and serum sodium and creatinine from paired samples.
- Interpret FENa with volume status, meds, and urine indices.
How the calculation works
FENa = (Urine Na × Serum Cr) ÷ (Serum Na × Urine Cr) × 100. Low (<1%) often prerenal; >2% often intrinsic — many exceptions.
Formula used by this tool — verify units before applying at the bedside.
Worked example
Limitations & clinical notes
- Diuretics invalidate FENa — consider FEUrea if on diuretics.
- Acute glomerulonephritis may present with low FENa despite intrinsic disease.
- Unreliable in advanced CKD, oliguria, and on diuretics — use clinical context and FEUrea when indicated.
References
Key sources: AKI differentiation indices.
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Disclaimer: This calculator is for educational and bedside double-check purposes only. It does not replace institutional protocols, prescriber orders, pharmacist verification, or clinical judgment. Always follow your local policies — especially for high-risk medications, infusions, pediatrics, and critical care. If the patient is acutely unwell, escalate immediately rather than relying on any calculator output.