Fractional Excretion of Sodium (FENa) Calculator

Calculate the Fractional Excretion of Sodium (FENa) to differentiate between prerenal and intrinsic renal failure. Enter urine sodium (U Na), urine creatinine (U Cr), plasma sodium (P Na), and plasma creatinine (P Cr) — and get back the FENa percentage along with a clinical interpretation: Pre-Renal, Intrinsic Renal, or Post-Renal.

Disclaimer: This tool is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any health-related decisions.

mEq/L

Sodium concentration measured in urine sample

mg/dL

Creatinine concentration measured in urine sample

mEq/L

Sodium concentration measured in blood plasma (serum)

mg/dL

Creatinine concentration measured in blood plasma (serum)

Results

FENa

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Clinical Interpretation

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Category

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Results Table

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Frequently Asked Questions

What does FENa stand for and what does it measure?

FENa stands for Fractional Excretion of Sodium. It measures the percentage of sodium filtered by the kidneys that is ultimately excreted in the urine. This ratio helps clinicians determine whether acute kidney injury (AKI) is caused by a pre-renal, intrinsic renal, or post-renal process.

What is the formula used to calculate FENa?

FENa is calculated as: FENa (%) = (U Na × P Cr) / (P Na × U Cr) × 100. Where U Na is urine sodium, P Cr is plasma creatinine, P Na is plasma sodium, and U Cr is urine creatinine.

How do I interpret FENa results?

A FENa below 1% typically indicates a pre-renal cause such as dehydration, congestive heart failure, or hepatorenal syndrome, where the kidneys are conserving sodium. A FENa between 1% and 3% suggests intrinsic renal failure (e.g., acute tubular necrosis). A FENa of 4% or greater may point toward post-renal obstruction or severe intrinsic renal damage.

Can FENa be unreliable in certain situations?

Yes. FENa can be falsely low in patients who have received diuretics, as diuretics artificially increase urinary sodium excretion. It should also be interpreted with caution in patients with contrast nephropathy, myoglobinuria, acute glomerulonephritis, and early urinary tract obstruction, where FENa may be less than 1% despite intrinsic injury.

What conditions can cause a low FENa (less than 1%)?

A low FENa (< 1%) is commonly seen in prerenal azotemia (e.g., hypovolemia, heart failure, cirrhosis), acute glomerulonephritis, hepatorenal syndrome, and acute partial urinary tract obstruction. In these states, the kidneys are actively reabsorbing sodium to restore perfusion.

What is the difference between FENa and FEUrea?

FENa measures sodium excretion and is the standard marker for differentiating prerenal from intrinsic AKI. FEUrea (fractional excretion of urea) is a useful alternative in patients on diuretics, since urea excretion is less affected by diuretic use. A FEUrea below 35% suggests prerenal AKI.

What are the normal units for each input value?

Urine sodium (U Na) and plasma sodium (P Na) are measured in mEq/L (milliequivalents per liter). Both urine creatinine (U Cr) and plasma creatinine (P Cr) are typically measured in mg/dL (milligrams per deciliter). Using consistent units ensures the creatinine values cancel out in the formula, yielding a valid percentage.

Should FENa be used alone to diagnose kidney failure type?

No. FENa is a helpful screening tool but should be used alongside clinical context, urine microscopy, patient history, and other lab values. A nephrologist's interpretation, along with assessment of volume status and medication history, is essential for accurate diagnosis of acute kidney injury.