Fractional Excretion of Sodium Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026
Disclaimer: This calculator is provided for informational and educational purposes only and does not constitute financial, medical, legal, or other professional advice. Always consult a qualified professional before making decisions based on these results.
Distinguishing why the kidneys aren't working properly
FENa helps differentiate prerenal causes of acute kidney injury (like dehydration or heart failure, where kidneys appropriately conserve sodium) from intrinsic renal causes (like acute tubular necrosis, where damaged tubules cannot reabsorb sodium normally).
Worked example
For urine sodium 20, serum creatinine 2.0, serum sodium 140, and urine creatinine 60:
FENa = (20 x 2.0) / (140 x 60) x 100 = 0.476% (Under 1% - suggests prerenal etiology)
This test is most reliable in oliguric patients not currently on diuretics, since diuretics significantly interfere with sodium handling and can produce misleading FENa results regardless of the underlying cause of kidney injury.
This is an informational tool only, not a substitute for clinical judgment - always consult a qualified healthcare provider for diagnosis and treatment decisions regarding kidney function.