Fractional Excretion of Sodium (FeNa) Calculator

Fractional Excretion of Sodium FeNa Explanation and Clinical Context
Fractional excretion of sodium FeNa quantifies the percentage of filtered sodium that is ultimately excreted in urine using simultaneously obtained urine and plasma measurements. The formula is FeNa equals Una times Pcr divided by Pna times Ucr all multiplied by one hundred to yield a percentage. FeNa is commonly used to help distinguish reduced renal perfusion states from intrinsic tubular injury. Values less than one percent generally indicate preserved tubular sodium reabsorption consistent with prerenal causes of azotemia. Values greater than two percent suggest impaired tubular reabsorptive function as seen in acute tubular injury. Values between one percent and two percent require integration with clinical context and other laboratory data such as urine microscopy and response to fluid challenge. Limitations include altered interpretation in patients receiving diuretic therapy and in some chronic kidney disease states. Always interpret FeNa together with history physical examination volume status and other laboratory data.

Reference
Hall JE. Guyton and Hall Textbook of Medical Physiology. Latest edition. Section on renal handling of sodium and water.
Pavian MR and Brenner BM. Approach to the patient with acute kidney injury. Clinical Nephrology review articles provide practical interpretation guidance.

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