# Fractional Excretion of Sodium (FENa)

Calculate the Fractional Excretion of Sodium (FENa) to differentiate prerenal from intrinsic acute kidney injury: FENa (%) = [U_Na × S_Cr] / [S_Na × U_Cr] × 100. In oliguric AKI, FENa <1% suggests prerenal azotemia (sodium-avid state: volume depletion, heart failure, hepatorenal syndrome), while FENa ≥1% suggests intrinsic injury (typically ATN). FENa >4% is sometimes cited as supportive of post-obstructive pathology. Caveats: unreliable on diuretics (use FEUrea <35%), contrast/sepsis ATN can present with low FENa, chronic CKD and glucosuria elevate FENa independently. Derived from Espinel CH, JAMA 1976. Most useful in oliguric AKI; interpret with full clinical context. Not medical advice.

> Canonical page: https://elysiatools.com/en/tools/fractional-excretion-sodium

- **Category:** Health

- **Keywords:** FENa, fractional excretion sodium, acute kidney injury, AKI, prerenal, ATN, acute tubular necrosis, urine sodium, nephrology, renal, oliguric

## Overview

The Fractional Excretion of Sodium (FENa) calculator estimates the percentage of filtered sodium excreted in urine using urine sodium, serum creatinine, serum sodium, and urine creatinine. In oliguric acute kidney injury, the result can help distinguish prerenal azotemia from intrinsic renal injury, but it must be interpreted with the full clinical context.

## Inputs

- **Urine Sodium U_Na (mmol/L)** (number): Urine sodium concentration in mmol/L. Must be ≥0.
- **Serum Creatinine S_Cr (mg/dL)** (number): Serum creatinine in mg/dL. Must be positive.
- **Serum Sodium S_Na (mmol/L)** (number): Serum sodium concentration in mmol/L. Must be positive.
- **Urine Creatinine U_Cr (mg/dL)** (number): Urine creatinine concentration in mg/dL. Must be positive.
- **Decimal Places** (number)

## When to use

- Assess oliguric acute kidney injury when prerenal azotemia and intrinsic renal injury are being considered.
- Review urine and serum laboratory values using the standard FENa formula.
- Support clinical interpretation alongside volume status, medications, comorbidities, and other kidney findings.

## How it works

- Enter urine sodium in mmol/L, serum creatinine in mg/dL, serum sodium in mmol/L, and urine creatinine in mg/dL.
- The calculator applies FENa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100.
- Choose the number of decimal places for the displayed result, from 0 to 8.
- The output reports the FENa percentage, category, and a related interpretation such as prerenal azotemia or intrinsic renal injury.

## Use cases

- Nephrology review of urine and serum measurements in oliguric acute kidney injury.
- Clinical education on the relationship between sodium handling and acute kidney injury categories.
- Quick documentation or verification of FENa calculations using standardized laboratory inputs.

## Frequently asked questions

### What does FENa measure?

FENa estimates the percentage of filtered sodium that is excreted in the urine.

### What does a FENa below 1% suggest?

In oliguric acute kidney injury, a value below 1% is consistent with prerenal azotemia or another sodium-avid state.

### What does a FENa of 1% or higher suggest?

A value of at least 1% is consistent with intrinsic renal injury, typically acute tubular necrosis.

### Can diuretics affect FENa interpretation?

Yes. Diuretics can make FENa unreliable; FEUrea may be considered instead, with a value below 35% often used in this context.

### Is this calculator medical advice?

No. FENa should be interpreted by a qualified clinician with the complete clinical picture.

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