What is FENa and what does it measure?

Fractional Excretion of Sodium (FENa) measures the percentage of filtered sodium excreted in urine. It helps differentiate prerenal azotemia from intrinsic kidney injury.

How do I interpret FENa results?

FENa 2% suggests intrinsic renal damage. Values 1-2% are indeterminate and require clinical correlation.

Does FENa work in patients on diuretics?

No, diuretics increase sodium excretion and falsely elevate FENa. Use Fractional Excretion of Urea (FEUrea) instead for patients on diuretics.

What labs are needed for FENa calculation?

You need four values: serum sodium, urine sodium, serum creatinine, and urine creatinine, ideally from samples collected at the same time.

When is FENa not reliable?

FENa may be unreliable with diuretic use, contrast nephropathy, rhabdomyolysis, acute glomerulonephritis, recent obstruction, and in patients with very low GFR.

What is the formula for FENa?

FENa = (Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine) × 100. The result is expressed as a percentage.

FENa Calculator

Calculate Fractional Excretion of Sodium to differentiate prerenal from intrinsic renal failure

Enter Lab Values

Sodium (Na)

Urine Na (mEq/L)

Plasma Na (mEq/L)

Creatinine (Cr)

Urine Cr (mg/dL)

Plasma Cr (mg/dL)

Patient is on diuretics

Formula

FENa = (U_Na × P_Cr) / (P_Na × U_Cr) × 100

FENa Interpretation Guide

FENa < 1%

Prerenal azotemia - kidneys retaining sodium

FENa 1-2%

Indeterminate - may be either prerenal or intrinsic

FENa ≥ 2%

Intrinsic renal disease - tubular dysfunction

When to Use FENa

• Acute kidney injury (AKI) evaluation
• Differentiating prerenal vs intrinsic causes
• Oliguria workup
• Assessing volume status

For Healthcare Professionals

This calculator is intended for use by healthcare professionals only. Clinical decisions should not be based solely on FENa values. Always consider the complete clinical picture.

About FENa Calculator - Fractional Excretion of Sodium

Calculate Fractional Excretion of Sodium (FENa) to differentiate prerenal from intrinsic kidney injury.

Our **FENa Calculator** helps differentiate between prerenal azotemia and intrinsic kidney injury using the fractional excretion of sodium. This critical clinical tool requires serum and urine sodium and creatinine values. For sepsis assessment, see our Sepsis Calculator.

FENa is particularly useful in acute kidney injury (AKI) workup. A FENa < 1% typically suggests prerenal azotemia (intact tubular function), while FENa > 2% suggests intrinsic renal damage. Values between 1-2% are indeterminate. Our calculator provides interpretation and clinical pearls.

**Clinical Note:** FENa interpretation has limitations. Diuretics, contrast, and certain conditions affect accuracy. Consider FEUrea in patients on diuretics. Always integrate results with clinical context. For renal-affected drug dosing, try our Vancomycin Calculator.

Key Features

FENa calculation

Prerenal vs intrinsic differentiation

Clinical interpretation

Limitations guidance

Related calculations

Reference ranges

Why Use This Tool?

Rapid AKI differentiation

Guide fluid management

Inform treatment decisions

Educational reference

Point-of-care calculation

Common Use Cases

Acute Kidney Injury Workup: Differentiate prerenal from intrinsic causes.

ICU Patient Assessment: Guide fluid resuscitation decisions.

Medical Education: Learn kidney function assessment.

Emergency Medicine: Rapid bedside calculation.

Technical Information

FENa = (UNa × PCr) / (PNa × UCr) × 100
Normal kidney tubules reabsorb >99% of filtered sodium
Prerenal: FENa < 1% (tubules working to conserve sodium)
Intrinsic: FENa > 2% (tubular damage impairs reabsorption)

How to Use

Enter serum sodium

Enter urine sodium

Enter serum creatinine

Enter urine creatinine

View FENa and interpretation