What FENa value indicates pre-renal AKI?

A FENa below 1% suggests the kidneys are avidly retaining sodium, which is consistent with pre-renal AKI due to volume depletion, heart failure, or hepatorenal syndrome. The tubules are intact and responding appropriately to low perfusion.

What FENa value indicates intrinsic (renal) AKI?

A FENa above 2% suggests tubular damage — most commonly acute tubular necrosis (ATN) — where the tubules can no longer reabsorb sodium effectively. Values between 1% and 2% are indeterminate and require clinical correlation.

Can FENa be used if the patient is on diuretics?

No. Diuretics (especially loop and thiazide diuretics) force sodium excretion and falsely elevate FENa, making it unreliable. Use the Fractional Excretion of Urea (FEUrea) instead, as urea excretion is less affected by diuretics.

What units should I use for creatinine?

Urine and serum creatinine must be in the same units (both mg/dL or both µmol/L) because the ratio cancels out the units. Mixing units will give an incorrect result. Sodium values are typically in mEq/L or mmol/L.

Are there conditions that cause a low FENa despite intrinsic AKI?

Yes. Early contrast nephropathy, myoglobinuria (rhabdomyolysis), acute glomerulonephritis, and acute interstitial nephritis can occasionally produce FENa <1% despite intrinsic injury. Always interpret FENa in the full clinical context.

Fractional Excretion of Sodium (FENa) Calculator

Name: Fractional Excretion of Sodium (FENa) Calculator
Availability: InStock
Author: Nham Vu

Calculate FENa to distinguish pre-renal from intrinsic (renal) acute kidney injury using urine and serum sodium and creatinine values.

Enter Lab Values

Use a spot urine and serum sample drawn at the same time.

Urine Sodium (U_Na) mEq/L

Serum Sodium (S_Na) mEq/L

Urine Creatinine (U_Cr) mg/dL

Serum Creatinine (S_Cr) mg/dL

Formula

FENa (%) = (U_Na × S_Cr) / (S_Na × U_Cr) × 100

Clinical Note

FENa is not reliable in patients on diuretics. Use FEUrea in that context. Always interpret in conjunction with clinical history, urine microscopy, and imaging.

Summary

Calculate FENa to distinguish pre-renal from intrinsic (renal) acute kidney injury using urine and serum sodium and creatinine values.

How it works

Collect a spot urine sample and a serum (blood) sample at the same time.
Enter the urine sodium (mEq/L or mmol/L) in the first field.
Enter the serum (plasma) sodium in the second field.
Enter the urine creatinine and serum creatinine (same units for both).
The calculator applies the formula: FENa (%) = (U_Na × S_Cr) / (S_Na × U_Cr) × 100.
Review the result and interpretation: <1% suggests pre-renal AKI; >2% suggests intrinsic renal AKI.

Use cases

Differentiating pre-renal azotemia from acute tubular necrosis (ATN) in hospitalized patients.
Evaluating oliguria in ICU patients to guide fluid resuscitation decisions.
Confirming suspected volume depletion as the cause of rising creatinine.
Monitoring AKI workup when diuretic therapy has not yet been started (FEUrea preferred if on diuretics).
Nephrology consult triage to prioritize patients who need urgent intervention.
Teaching and reviewing renal physiology in medical education settings.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu