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FeNa Calculator

FeNa calculator computes Fractional Excretion of Sodium to differentiate prerenal from intrinsic acute kidney injury. A FeNa below 1% suggests prerenal causes, while above 2% suggests intrinsic renal damage such as acute tubular necrosis.

Enter Values

Serum Sodium (mEq/L)

Serum Creatinine (mg/dL)

Urine Sodium (mEq/L)

Urine Creatinine (mg/dL)

Result

Enter values above and click Calculate to see your result.

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Formula

Core Formula

\text{FENa} = \frac{U_{Na} \times S_{Cr}}{S_{Na} \times U_{Cr}} \times 100

How it works: Measures the percentage of sodium filtered by the kidney that is excreted in the urine.

Worked Example

Serum: Na 140, Cr 1.5; Urine: Na 10, Cr 50 -> FeNa ~ 0.14%.

Understanding Fractional Excretion of Sodium (FeNa)

FeNa measures the percentage of filtered sodium that is excreted in the urine. It is one of the most useful tests for differentiating prerenal from intrinsic causes of acute kidney injury.

Formula: FeNa = (Urine Na x Plasma Cr) / (Plasma Na x Urine Cr) x 100
FeNa below 1%: prerenal AKI (kidneys aggressively reabsorbing sodium due to low perfusion)
FeNa 1-2%: mixed or indeterminate, requires clinical correlation
FeNa above 2%: intrinsic renal damage such as Acute Tubular Necrosis (ATN)
Diuretics invalidate FeNa; use Fractional Excretion of Urea (FeUrea) instead in those patients

This calculator is for educational purposes only. FeNa should be interpreted in clinical context alongside urine microscopy, imaging, and other renal markers.

You can also calculate changes using our BUN Creatinine Ratio Calculator or eGFR Calculator (MDRD).

Frequently Asked Questions

What does a FeNa below 1% mean?

A FeNa below 1% suggests a prerenal cause of AKI (such as dehydration or heart failure), where the kidneys are aggressively reabsorbing sodium to preserve volume.

What does a FeNa above 2% mean?

A FeNa above 2% suggests intrinsic renal damage, most commonly Acute Tubular Necrosis (ATN), where damaged tubules can no longer reabsorb sodium effectively.

What does a FeNa between 1% and 2% indicate?

Values between 1% and 2% fall in an indeterminate zone and may represent a mixed picture or early transition from prerenal to intrinsic AKI. Clinical correlation is essential.

Can diuretics affect FeNa?

Yes. Diuretics increase sodium excretion, making a prerenal state appear intrinsic. In patients on diuretics, the Fractional Excretion of Urea (FeUrea) is preferred: below 35% suggests prerenal, above 50% suggests intrinsic.

When is FeNa most reliable?

FeNa is most reliable in oliguric patients (urine output below 0.5 mL/kg/hr) who are not on diuretics. It is less accurate in chronic kidney disease, contrast nephropathy, and early obstruction.

Is this calculator a substitute for medical advice?

No. This tool is for educational purposes only. Always consult a qualified healthcare professional for diagnosis and management of acute kidney injury.

Is it possible to embed the FeNa Calculator on another website?

Yes, embedding the FeNa Calculator is free. Hit the "Embed" button on this page, adjust the width, height, and theme, then grab the iframe code. It works on WordPress, Wix, Squarespace, Shopify, and plain HTML pages. No registration needed. Full instructions at calculory.com/services/embed-calculators.

Medical Disclaimer

This calculator is provided for informational and educational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions based on these results. Do not disregard or delay seeking medical advice because of information obtained from this tool.

AI Assistant

Ask about this calculator

I can help you understand the fena calculator formula, interpret your results, and answer follow-up questions.

Try asking

Our AI assistant provides general estimates and advice only. Please consult a qualified healthcare professional before making any health-related decisions.

Related Calculators

BUN Creatinine Ratio Calculator eGFR Calculator (MDRD)

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