Fractional sodium excretion

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:

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(Sodium_urinary×Flow rate_urinary)÷(Sodium_plasma×(Creatinine_urinary×Flow rate_urinary÷Creatinine_plasma)×100

Sodium (mmol/l) Creatinine (mg/dl)

The flow rates cancel out in the above equation, simplifying to the standard equation:

(Sodium_urinary×Creatinine_plasma)÷(Sodium_plasma×Creatinine_urinary)×100

This can also be rearranged in a more intuitive ratio of fractions:

(Sodium_urinary÷Creatinine_urinary)÷(Sodium_plasma÷Creatinine_plasma)×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.^[2]

References