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Fractional Excretion of Sodium (FENa) Calculator

Calculate FENa to differentiate prerenal from intrinsic renal failure. FENa = (UNa × PCr) / (PNa × UCr) × 100. FENa below 1% suggests prerenal azotemia; above 2% suggests intrinsic renal disease.

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Calculate Fractional Excretion of Sodium to differentiate prerenal azotemia from ATN in acute kidney injury.

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Why: Calculate Fractional Excretion of Sodium to differentiate prerenal azotemia from ATN in acute kidney injury.

How: Enter your values in the calculator fields below, keep units consistent, then run the calculation to see results and any step-by-step work shown on this page.

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Spot urine
mEq/L
Spot urine
mg/dL
Plasma
mEq/L
Plasma
mg/dL
Enter valid Urine Na, Urine Cr, Serum Na, and Serum Cr (all positive)

For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.

FENa differentiates prerenal from intrinsic renal failure in AKI. Formula: FENa = (UNa × PCr) / (PNa × UCr) × 100. FENa below 1% suggests prerenal azotemia; above 2% suggests intrinsic renal disease (ATN).

<1%
Prerenal
1-2%
Indeterminate
>2%
ATN
Spot
Urine OK

Sources: KDIGO AKI Guidelines, Nephrology textbooks.

Key Takeaways

  • • FENa <1%: Prerenal (volume depletion, heart failure, cirrhosis, early sepsis)
  • • FENa >2%: Intrinsic renal/ATN (ischemic, nephrotoxic)
  • • FENa 1-2%: Indeterminate; use clinical context, FEUrea if on diuretics
  • • Unreliable with diuretics—use FEUrea instead

Did You Know?

🔢 FENa uses spot urine—no timed collection needed
📊 FEUrea &lt;35% = prerenal, &gt;50% = ATN (use with diuretics)
💡 Contrast nephropathy can have low FENa in first 24h
🌍 Sepsis-associated AKI often has low FENa despite ATN
📈 BUN/Cr &gt;20:1 supports prerenal component
🎯 Urine microscopy (muddy brown casts) suggests ATN

How Does FENa Work?

Formula

FENa = (UNa × PCr) / (PNa × UCr) × 100

Compares filtered vs excreted sodium. Prerenal: kidney conserves Na (low FENa). ATN: tubules damaged, Na wasted (high FENa).

Sample Collection

Simultaneous spot urine and serum. Creatinine corrects for urine concentration.

Caveats

Diuretics falsely elevate FENa. Use FEUrea. Exceptions: contrast (early), rhabdo, sepsis can have low FENa despite ATN.

Expert Tips

Use FEUrea when patient on loop or thiazide diuretics
Correlate with BUN/Cr, urine osmolality, and microscopy
Low FENa with hypervolemia suggests cardiorenal or hepatorenal syndrome
Serial monitoring if FENa indeterminate; reassess volume status

FENa Interpretation

FENaCategoryTypical Causes
<1%PrerenalDehydration, HF, cirrhosis, sepsis
1-2%IndeterminateTransition, early ATN, diuretic
>2%ATNIschemic, nephrotoxic, contrast

Frequently Asked Questions

What is FENa?

Fractional Excretion of Sodium. FENa = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100. Differentiates prerenal (&lt;1%) from ATN (&gt;2%).

When is FENa unreliable?

With diuretics, contrast, CKD, or low urine output. Use FEUrea instead when on loop diuretics.

What does FENa &lt;1% mean?

Prerenal azotemia - kidney conserving sodium. Think hypovolemia, heart failure, cirrhosis, early sepsis.

What does FENa &gt;2% mean?

Intrinsic renal (ATN) - tubular injury. Sodium not reabsorbed. Think ischemic or nephrotoxic injury.

FENa 1-2%?

Indeterminate. May be early ATN, resolving prerenal, or diuretic effect. Use clinical context.

Do I need a timed urine collection?

No, a spot urine sample is sufficient. The creatinine in the formula corrects for urine concentration. Collect urine and serum simultaneously.

Key Statistics

1%
Prerenal cutoff
2%
ATN cutoff
35%
FEUrea prerenal
50%
FEUrea ATN

Official Data Sources

KDIGO AKI Guidelines ↗
AKI definition and management
UpToDate FENa ↗
FENa clinical use
PubMed FENa ↗
FENa literature
Nephrology Society ↗
NKF patient resources

⚠️ Disclaimer: This calculator is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.

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