Fractional sodium excretion

The fractional excretion of sodium (FE_Na) is the percentage of the sodium filtered by the kidney which is excreted in the urine. 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 reabsorption. Therefore, the urinary and plasma concentrations of sodium must be compared to get an accurate picture of renal clearance. In clinical use, the fractional excretion of sodium can be calculated as part of the evaluation of acute kidney failure in order to determine if hypovolemia or decreased effective circulating plasma volume is a contributor to the kidney failure.

FE_Na is calculated in two parts—figuring out how much sodium is excreted in the urine, and then finding its ratio to the total amount of sodium that passed through (aka "filtered by") the kidney.

First, the actual amount of sodium excreted is calculated by multiplying the urine sodium concentration by the urinary flow rate. This is the numerator in the equation. The denominator is the total amount of sodium filtered by the kidneys. This is calculated by multiplying the plasma sodium concentration by the glomerular filtration rate calculated using creatinine filtration. This formula is represented mathematically as:

[(Sodium_urinary × Flow rate_urinary) ÷ (Sodium_plasma) x (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:

${\displaystyle FE_{Na}{=}100\times {\frac {\rm {sodium_{urinary}\times creatinine_{plasma}}}{\rm {sodium_{plasma}\times creatinine_{urinary}}}}}$

...
Wikipedia