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Fractional excretion of sodium, urea, and other molecules in acute kidney injury (acute renal failure)

Rasheed A Balogun, MD
Mark D Okusa, MD
Section Editor
Paul M Palevsky, MD
Deputy Editor
Alice M Sheridan, MD


The fractional excretion of sodium (FENa) measures the percent of filtered sodium that is excreted in the urine. This calculation is widely used to help differentiate prerenal disease (decreased renal perfusion) from acute tubular necrosis (ATN) as the cause of acute kidney injury (AKI, formerly called acute renal failure). (See "Etiology and diagnosis of prerenal disease and acute tubular necrosis in acute kidney injury in adults", section on 'Evaluation and diagnosis'.)

As described below, there are a number of limitations to the use of FENa in patients with AKI. These limitations include the variability of the threshold FENa that is used to define prerenal disease, the presence of multiple causes of a low FENa other than prerenal disease, and errors associated with the use of measures of sodium excretion in the setting of renal salt wasting (as with diuretic therapy). Thus, the etiology of AKI should be based upon the totality of the presentation (history, clinical examination, urine microscopy, and, when appropriate, response to volume resuscitation), not just on the FENa. When diuretics may still be active and the FENa is not low, the fractional excretion of urea (FEUrea) may be helpful. (See 'Limitations of fractional excretion of sodium' below and 'Urea' below.)

Although not yet in widespread clinical use, novel urinary and serum biomarkers to assess intrinsic tubular injury are being evaluated [1-3]. (See "Etiology and diagnosis of prerenal disease and acute tubular necrosis in acute kidney injury in adults", section on 'Evaluation and diagnosis'.)

Formula — The filtered sodium load can be determined from the product of the glomerular filtration rate (GFR; as estimated from the creatinine clearance) and the serum sodium concentration (SNa). On the other hand, urinary sodium excretion is equal to the product of the urine sodium concentration (UNa) and the urine flow rate (V). Thus:

                                                   quantity of Na+ excreted
         FENa, percent    =      ——————————————————    x    100
                                                     quantity of Na+ filtered

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Literature review current through: Oct 2017. | This topic last updated: May 26, 2017.
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