The FENa is a calculated value used in determining the etiology of acute renal failure (ARF) or the volume status of a patient. The value is calculated from the serum sodium and creatinine and also the urinary values for sodium and creatinine. The urinary values are obtained from a random urine sample collected while the patient has not received any recent diuretics. The FENa is calculated as follows:

FENa values less than 1 are consistent with a prerenal etiology or dehydration. This implies that the kidneys are being hypoperfused secondary to either low plasma volume or impaired cardiac output.

FENa values greater than 2 are consistent with renal parenchymal pathology (intrinsic renal disease). Acute renal failure with a high FENa (>2) can be secondary to acute interstitial nephritis (AIN) or acute tubular necrosis (ATN). ATN can be secondary to prolonged renal hypoperfusion of any cause, aminoglycoside nephrotoxicity, iodinated radiocontrast dye nephrotoxicity or myoglobin induced nephrotoxicity seen in patients with rhabdomyolysis.