Fractional Excretion of Sodium (FENa)

Fractional Excretion of Sodium (FENa) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Health
Input types: number
Output type: json
Sample coverage: 4
API ready: Yes

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.

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

1Enter urine sodium in mmol/L, serum creatinine in mg/dL, serum sodium in mmol/L, and urine creatinine in mg/dL.
2The calculator applies FENa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100.
3Choose the number of decimal places for the displayed result, from 0 to 8.
4The 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.

Examples

1. Low FENa consistent with prerenal azotemia

Clinician reviewing an oliguric AKI case

Background

The patient has urine sodium of 20 mmol/L, serum creatinine of 1.5 mg/dL, serum sodium of 140 mmol/L, and urine creatinine of 120 mg/dL.

Problem

Calculate FENa and review whether the result is consistent with a sodium-avid state.

How to use

Enter 20 for urine sodium, 1.5 for serum creatinine, 140 for serum sodium, and 120 for urine creatinine. Set decimal places to 4.

urineSodium=20; serumCreatinine=1.5; serumSodium=140; urineCreatinine=120; decimalPlaces=4

Outcome

The calculator returns FENa = 0.1786%, below 1%, consistent with prerenal azotemia in the appropriate clinical context.

2. FENa in the intrinsic injury range

Medical trainee studying AKI classification

Background

The laboratory values are urine sodium 60 mmol/L, serum creatinine 2.0 mg/dL, serum sodium 135 mmol/L, and urine creatinine 60 mg/dL.

Problem

Use the standard formula to calculate FENa and identify the reported category.

How to use

FAQ

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.

urineSodium=60; serumCreatinine=2.0; serumSodium=135; urineCreatinine=60; decimalPlaces=4

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Low FENa consistent with prerenal azotemia

2. FENa in the intrinsic injury range

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