When is FENa unreliable?

Diuretics (they block sodium reabsorption and raise FENa even in prerenal states), CKD (impaired baseline sodium handling), glycosuria (osmotic diuresis), and after recent fluids or radiocontrast. In these situations FEUrea (<35% prerenal, ≥35% ATN) is the preferred test.

Why is FENa <1% in prerenal azotemia?

When kidney blood flow falls, the tubules work harder to conserve sodium — reabsorbing nearly all filtered sodium. So urine sodium falls and FENa drops below 1%. In tubular injury, the damaged tubules can't reabsorb sodium, and FENa rises above 2%.

Can FENa be used in chronic kidney disease?

Not reliably. CKD kidneys have high baseline FENa (tubular dysfunction), so a high FENa doesn't distinguish ATN from baseline in CKD patients. In CKD, the diagnosis of AKI relies on creatinine trends and urine sediment rather than FENa.

Acute Kidney Injury

Fractional Excretion of Sodium (FENa)

Triage AKI: prerenal vs intrinsic tubular injury. Updated 2026-08.

Fractional Excretion of Sodium (FENa)

Triage AKI: prerenal vs intrinsic tubular injury

Urine Sodium(mEq/L)

Serum Sodium(mEq/L)

Urine Creatinine(mg/dL)

Serum Creatinine(mg/dL)

Enter your values above to see the result.

For education and screening only — not a substitute for professional medical assessment. Results should be interpreted by your healthcare provider.

Sources & References

Espinel CH. The FENa test. JAMA 1976;236:579-581
Macedo E, Mehta RL. Fractional excretion of sodium and urea. Clin J Am Soc Nephrol 2010

How to Use

1Enter urine sodium, serum sodium, urine creatinine, and serum creatinine from the same time point.
2Interpret: <1% prerenal (volume depletion), >2% ATN.
3If the patient is on diuretics, FENa is unreliable — use the FEUrea calculator.

Understanding Results

Prerenal pattern (<1%)

Classic prerenal azotemia — volume depletion or reduced kidney perfusion. The kidneys are intact and holding sodium; fluids usually restore function.

Borderline (1-2%)

Indeterminate zone — common with diuretics, early ATN, CKD, or glycosuria. Consider FEUrea for a cleaner signal.

Intrinsic renal (ATN) pattern (>2%)

Suggests tubular injury (ATN). Exclude the confounders first: diuretics, chronic kidney disease, and glucose in urine all falsely raise FENa.

Frequently Asked Questions

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FEUrea (Fractional Excretion of Urea)

Distinguish prerenal from ATN — even on diuretics

Learn more

AKI Stage Checker (KDIGO 2012)

Stage acute kidney injury from creatinine and urine output

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eGFR Calculator (CKD-EPI 2021)

Estimate glomerular filtration rate and check your CKD stage

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Creatinine Clearance (Cockcroft-Gault)

Estimate creatinine clearance for drug dosing

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Explore related tools & guides

Tool: FEUrea (Fractional Excretion of Urea)Tool: AKI Stage Checker (KDIGO 2012)Topic: Acute Kidney Injury (AKI): The Kidney Emergency Explained Topic: AKI Recovery: What Happens After Acute Kidney Injury

Acute Kidney Injury

Fractional Excretion of Sodium (FENa)

Triage AKI: prerenal vs intrinsic tubular injury. Updated 2026-08.

Fractional Excretion of Sodium (FENa)

Triage AKI: prerenal vs intrinsic tubular injury

Urine Sodium(mEq/L)

Serum Sodium(mEq/L)

Urine Creatinine(mg/dL)

Serum Creatinine(mg/dL)

Enter your values above to see the result.

For education and screening only — not a substitute for professional medical assessment. Results should be interpreted by your healthcare provider.

Sources & References

Espinel CH. The FENa test. JAMA 1976;236:579-581
Macedo E, Mehta RL. Fractional excretion of sodium and urea. Clin J Am Soc Nephrol 2010

How to Use

1Enter urine sodium, serum sodium, urine creatinine, and serum creatinine from the same time point.
2Interpret: <1% prerenal (volume depletion), >2% ATN.
3If the patient is on diuretics, FENa is unreliable — use the FEUrea calculator.

Understanding Results

Prerenal pattern (<1%)

Classic prerenal azotemia — volume depletion or reduced kidney perfusion. The kidneys are intact and holding sodium; fluids usually restore function.

Borderline (1-2%)

Indeterminate zone — common with diuretics, early ATN, CKD, or glycosuria. Consider FEUrea for a cleaner signal.

Intrinsic renal (ATN) pattern (>2%)

Suggests tubular injury (ATN). Exclude the confounders first: diuretics, chronic kidney disease, and glucose in urine all falsely raise FENa.

Frequently Asked Questions

Free 1-Year Access

The Clinical OS for Dialysis Centers

ZuvFlo unifies chair scheduling, session charting, machine integration, billing, and NABH-ready documentation — the operating system behind modern dialysis operations.

Schedule a Demo View Pricing

No credit card required • Setup in under 2 hours • Cancel anytime

Fractional Excretion of Sodium (FENa)

Fractional Excretion of Sodium (FENa)

Sources & References

How to Use

Understanding Results

Frequently Asked Questions

The Clinical OS for Dialysis Centers

More Calculators

FEUrea (Fractional Excretion of Urea)

AKI Stage Checker (KDIGO 2012)

eGFR Calculator (CKD-EPI 2021)

Creatinine Clearance (Cockcroft-Gault)

Explore related tools & guides

Educational information only

Sources and methodology

Fractional Excretion of Sodium (FENa)

Fractional Excretion of Sodium (FENa)

Sources & References

How to Use

Understanding Results

Frequently Asked Questions

The Clinical OS for Dialysis Centers

More Calculators

FEUrea (Fractional Excretion of Urea)

AKI Stage Checker (KDIGO 2012)

eGFR Calculator (CKD-EPI 2021)

Creatinine Clearance (Cockcroft-Gault)

Explore related tools & guides

Educational information only

Sources and methodology