FENa helps distinguish two causes of acute kidney injury — reduced blood flow to the kidneys versus direct kidney tissue damage — which matters in critically ill patients, including those in respiratory failure or on ventilators.
Prerenal — reduced kidney perfusion (often fluid-responsive).
Indeterminate.
Intrinsic damage — e.g. acute tubular necrosis.
Critically ill ventilated or septic patients are at high risk of acute kidney injury, and distinguishing the cause changes management — for example fluid resuscitation for a prerenal cause versus supportive care for tubular injury.
Under 1% suggests a prerenal cause (reduced kidney blood flow); over 2% suggests intrinsic kidney damage such as acute tubular necrosis. 1–2% is indeterminate.
In patients on diuretics, which force sodium excretion and raise FENa artificially. The fractional excretion of urea (FEUrea) is used instead in that setting.
FENa (%) = [(urine Na × serum creatinine) ÷ (serum Na × urine creatinine)] × 100.