Fractional Excretion of Sodium (FENa) Calculator

FENa's formula, (urine Na x plasma creatinine) / (plasma Na x urine creatinine) x 100, has urine sodium and plasma creatinine in the numerator. Since the equation is a product with no other terms, both numerator values push the result in the same direction -- and because the formula multiplies the pair with no asymmetry between them, nudging urine sodium up by some fraction raises FENa by that exact same fraction as nudging plasma creatinine up by the same amount would.

Diuretics work by forcing the kidneys to excrete more sodium, regardless of whether the underlying cause of acute kidney injury is prerenal or intrinsic renal. That artificially inflates urine sodium and pushes FENa toward the "intrinsic renal" range even in a patient whose kidneys are actually responding appropriately to reduced blood flow. Fractional excretion of urea (FEUrea) is used instead in diuretic-treated patients because urea handling is affected much less by diuretics.

A FENa below 1% suggests a prerenal cause of acute kidney injury -- the kidneys are structurally intact and are appropriately conserving sodium in response to reduced blood flow or volume, as seen in dehydration, blood loss, or heart failure. This typically points toward volume repletion as the treatment, in contrast to a high FENa suggesting intrinsic kidney damage that volume alone won't fix.

A FENa above 2% suggests an intrinsic renal cause of acute kidney injury, most commonly acute tubular necrosis, where the kidney tubules themselves are damaged and can no longer reabsorb filtered sodium normally, so more of it ends up in the urine. This is a structural kidney problem rather than a blood-flow problem, and it generally does not improve with fluid resuscitation the way a prerenal picture does.