Fractional sodium excretion
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Overview
The fractional excretion of sodium (FENa) is a measure of the percentage of sodium excreted in the urine versus the sodium reabsorbed by the kidney. It is measured in terms of plasma and urine sodium, rather than by the interpretation of urinary sodium concentration alone, as urinary sodium concentrations can vary with water resorption. Therefore the urinary and plasma concentrations of sodium must be compared to get an accurate picture of renal clearance. FENa can be calculated by multiplying the plasma sodium concentration by the glomerular filtration rate. It may also be calculated by multiplying the urine sodium concentration by the urinary flow rate. This translates into the formula:
(Sodiumurinary×Flow rateurinary)÷(Sodiumplasma×(Creatinineurinary×Flow rateurinary÷Creatinineplasma)×100
Sodium (mmol/l) Creatinine (mg/dl)
The flow rates cancel out in the above equation, simplifying to the standard equation:
(Sodiumurinary×Creatinineplasma)÷(Sodiumplasma×Creatinineurinary)×100
This can also be rearranged in a more intuitive ratio of fractions:
(Sodiumurinary÷Creatinineurinary)÷(Sodiumplasma÷Creatinineplasma)×100
Interpretation
FENa is an accurate screening test for differentiating prerenal failure versus acute tubular necrosis. A value below 1 percent suggests prerenal disease, as the physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hypovolemia. Values above 2 percent usually indicate acute tubular necrosis: either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypervolemia resulting in the normal response of sodium wasting. Values between 1 and 2 may be seen in either disorder. In renal tract obstruction, values may be either higher or lower than 1%.[1]
Alternatives
Fractional excretion of other substances can be measured to determine renal clearance including urea, uric acid, and lithium. These can be used in patients undergoing diuretic therapy, where the urinary sodium concentrations may be higher despite possible prerenal pathology.[1]
References
Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
