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Fractional Excretion of Sodium

The fractional excretion of sodium (FENa) measures the percent of filtered sodium that is excreted in the urine. The FENa is the most accurate screening test to help differentiate between prerenal disease and acute tubular necrosis (ATN), the two most common causes of acute kidney injury. FENa is generally more accurate than the urine sodium concentration among patients with acute kidney injury because it directly measures sodium handling.

FENa%= (UNa x PCr) / (PNa x UCr)

A value of FENa below 1% suggests prerenal disease, where the reabsorption of almost all of the filtered sodium represents an appropriate response to decreased renal perfusion. A value above 2% usually indicates acute tubular necrosis (ATN). A value between 1 and 2 percent may be seen with either disorder. The relatively high FENa in ATN can be due to one or both of the following factors: inappropriate sodium wasting due to tubular damage and an appropriate response to volume expansion. The former is most likely to be important early in the disease when nephrons that are still filtering have impaired tubular function. In comparison, volume expansion may be more important in patients with established ATN and normal tissue perfusion. These cut-off values apply only to patients with advanced renal failure.

In addition to prerenal disease, FENa can be less than 1% in patients with postischemic ATN, ATN superimposed upon chronic prerenal disease, kidney injury due to radiocontrast media or heme pigments, acute glomerulonephritis or vasculitis, some cases of acute interstitial nephritis, and rarely acute urinary tract obstruction.

FENa and urine sodium concentration are difficult to interpret with concurrent diuretic therapy. The fractional excretion of urea (FEurea) may be most useful in this setting, although inconsistent results have been reported. In general, the FEurea is between 50 to 65 percent in ATN and usually below 35 percent in prerenal disease.

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