FENa Calculator — Fractional Excretion of Sodium

Calculate the fractional excretion of sodium (FENa) to differentiate pre-renal from intrinsic renal acute kidney injury.

Urine Sodium (mEq/L)

Serum Creatinine (mg/dL)

Serum Sodium (mEq/L)

Urine Creatinine (mg/dL)

How to Use the FENa Calculator

Enter urine sodium (from spot urine) in mEq/L.
Enter serum creatinine in mg/dL.
Enter serum sodium in mEq/L.
Enter urine creatinine (from spot urine) in mg/dL.
Click Calculate to see FENa and interpretation.

Use Cases

•Differentiating pre-renal AKI from acute tubular necrosis (ATN).
•Evaluating oliguria in hospitalized or ICU patients.
•Teaching renal physiology and AKI classification.

Formula

FENa (%) = (Urine Na × Serum Creatinine) / (Serum Na × Urine Creatinine) × 100. FENa < 1% = pre-renal; > 2% = intrinsic renal.

Frequently Asked Questions

What does FENa tell us?

FENa measures what fraction of filtered sodium is excreted in the urine. Pre-renal AKI (dehydration, heart failure) causes avid sodium retention (FENa < 1%). Intrinsic renal injury (ATN) impairs reabsorption, leading to FENa > 2%.

When is FENa unreliable?

FENa is unreliable in patients on diuretics (use FEUrea instead), contrast nephropathy, myoglobinuria, or early obstruction. Clinical context is essential.

Disclaimer

For educational purposes only. FENa is one component of AKI evaluation and must be interpreted alongside history, physical exam, urine microscopy, and other labs. Not a substitute for clinical judgment.