Fractional Excretion of Sodium (FENa)

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Differentiate between prerenal and intrinsic renal causes of kidney injury.

Lab Parameters

Serum Sodium (mEq/L)

Urine Sodium (mEq/L)

Serum Creatinine (mg/dL)

Urine Creatinine (mg/dL)

Diagnostic Analysis

Calculated FENa0.21%

Probable CausePrerenalDifferential

Reference Guide

Prerenal< 1%

Indeterminate1% - 2%

Intrinsic (ATN)> 2%

About this calculator

Overview

FENa is the fraction of sodium filtered by the kidney that is eventually excreted in the urine. It is calculated using serum and urine sodium and creatinine levels. It is primarily used to help distinguish between prerenal azotemia and acute tubular necrosis (ATN).

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Pro Tips

Samples for serum and urine should be collected at approximately the same time.
Ensure the patient has not received diuretics in the last 12-24 hours.
Clinical context (like volume status) is vital for interpreting the result.

Fun Facts

"A FENa < 1% suggests prerenal causes (dehydration, decreased blood flow)."
"A FENa > 2% suggests intrinsic renal damage (like ATN)."
"FENa is most useful in oliguric patients (urine output < 400 mL/day)."
"Diuretics significantly interfere with FENa accuracy (consider using FEUrea instead)."

Formula: FENa (%) = [ (PCr × UNa) / (PNa × UCr) ] × 100