FENa Calculator — Fractional Excretion of Sodium for AKI Etiology
Calculate Fractional Excretion of Sodium (FENa) from paired serum and urine electrolytes to differentiate prerenal azotemia from intrinsic acute tubular necrosis (ATN).
FENa Calculator
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This calculator is for informational and educational purposes. Results should be interpreted with clinical judgment and local guidelines.
What is FENa (Fractional Excretion of Sodium)?
Fractional Excretion of Sodium (FENa) is the percentage of sodium filtered by the kidney glomeruli that is excreted in the urine. It measures the kidney's tubular reabsorptive capacity to differentiate prerenal acute kidney injury (in which intact tubules aggressively reabsorb sodium to preserve intravascular volume, resulting in FENa < 1%) from intrinsic acute tubular necrosis (ATN, in which damaged tubules cannot reabsorb sodium, resulting in FENa > 2%). FENa requires simultaneous measurement of serum sodium, serum creatinine, urine sodium, and urine creatinine.
FENa (%) = [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100. FENa < 1% = Prerenal Azotemia. FENa > 2% = Intrinsic ATN. FENa 1–2% = Indeterminate / Mixed.
When to Use the FENa Calculator
- Diagnostic workup of acute kidney injury (AKI) with acute rise in serum creatinine in oliguric or non-oliguric patients.
- Differentiating prerenal azotemia (dehydration, volume depletion, heart failure, cirrhosis) from intrinsic Acute Tubular Necrosis (ischemic or nephrotoxic ATN).
- Evaluating oliguria in ICU patients after cardiac surgery, trauma, or septic shock.
FENa Formula & Parameter Breakdown
FENa Standard Percentage Formula
FENa (%) = [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100
UNa = 10 mEq/L, UCr = 100 mg/dL, SNa = 140 mEq/L, SCr = 2.0 mg/dL → FENa = [(10 × 2) / (140 × 100)] × 100 = (20 / 14000) × 100 = 0.14% (Prerenal!)
Variables
Urine Sodium (UNa) (mEq/L)
Spot urine sodium concentration.
Serum Sodium (SNa) (mEq/L)
Serum sodium level measured at the same time.
Urine Creatinine (UCr) (mg/dL)
Spot urine creatinine concentration.
Serum Creatinine (SCr) (mg/dL)
Serum creatinine level measured at the same time.
If patient has received diuretics (furosemide), FENa is falsely elevated! Use Fractional Excretion of Urea (FEUrea) instead.
FENa Clinical Interpretation Table
FENa distinguishes intact tubular sodium conservation from tubular injury.
| Range | Classification | Health Risk |
|---|---|---|
| < 1.0% | Prerenal Azotemia | Intact tubular function; renal hypoperfusion (dehydration, heart failure, sepsis). |
| 1.0%–2.0% | Indeterminate / Overlapping | Mixed picture; early ATN, acute glomerulonephritis, or contrast nephropathy. |
| > 2.0% | Intrinsic Acute Tubular Necrosis (ATN) | Tubular epithelial damage; inability to reabsorb sodium. |
FENa can be falsely < 1% in early contrast nephropathy, acute glomerulonephritis, pigment nephropathy (rhabdomyolysis), or severe CKD.
Limitations & Clinical Warnings
Diuretic use invalidates FENa! Furosemide inhibits tubular sodium reabsorption, causing FENa > 1% even in prerenal states. Use FEUrea (< 35% = prerenal) when patient is on diuretics.
Not reliable in chronic kidney disease (CKD), where baseline FENa may be elevated.
Spot urine must be collected simultaneously with serum blood draw.
Medical Disclaimer
FENa is a diagnostic aid. Integrate with clinical volume status assessment, urine sediment microscopy, and imaging.
References & Citations
- Espinel CH. “The FENa test. Use in the differential diagnosis of acute renal failure”. JAMA. 1976. 236(6): 579–581. PMID: 947239
- Carvounis CP, Nisar S, Guro-Razuman S. “Significance of fractional excretion of urea in the differential diagnosis of acute renal failure”. Kidney International. 2002. 62(6): 2223–2229. PMID: 12427149
Medically Reviewed
Reviewed by CliniqWise Clinical Advisory Board — Nephrology Panel · Last updated: 2026-08-27 · Next review: 2027-02-27
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