Therefore, the urinary and plasma concentrations of sodium must be compared to get an accurate picture of kidney 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.
[citation needed] First, the actual amount of sodium excreted is calculated by multiplying the urine sodium concentration by the urinary flow rate (UFR).
The denominator is the total amount of sodium filtered by the kidneys.
The flow rates then cancel out, simplifying to the standard equation:[1]
FENa can be useful in the evaluation of acute kidney failure in the context of low urine output.
Low fractional excretion indicates sodium retention by the kidney, suggesting pathophysiology extrinsic to the urinary system such as volume depletion or decrease in effective circulating volume (e.g. low output heart failure).
Higher values can suggest sodium wasting due to acute tubular necrosis or other causes of intrinsic kidney failure.
While the above values are useful for older children and adults, the FENa must be interpreted more cautiously in younger pediatric patients due to the limited ability of immature tubules to reabsorb sodium maximally.
Thus, in term neonates, a FENa of <3% represents volume depletion, and a FENa as high as 4% may represent maximal sodium conservation in critically ill preterm neonates.
[4][5] The FENa may also be spuriously elevated in children with adrenal insufficiency or pre-existing kidney disease (such as obstructive uropathy) due to salt wasting.
[6] The FENa is generally less than 1% in patients with hepatorenal syndrome and acute glomerulonephropathy.
Although often reliable at discriminating between prerenal azotemia and acute tubular necrosis, the FENa has been reported to be <1% occasionally with oliguric and nonoliguric acute tubular necrosis, urinary tract obstruction, acute glomerulonephritis, renal allograft rejection, sepsis, and drug-related alterations in renal hemodynamics.
Fractional excretion of other substances can be measured to determine kidney clearance including urea, uric acid, and lithium.
Thus, the urinary sodium concentration and FENa may be higher in patients receiving diuretics in spite of prerenal pathology.