Fractional excretion of sodium (FENa) is calculated as

The fractional excretion of sodium (FeNa) is the fraction of the filtered sodium load that is reabsorbed by the tubules FENa relates sodium clearance to creatinine clearance. As such, it depends on sodium intake, effective intravascular volume, GFR, diuretic intake, and intact tubular reabsorptive mechanisms. With prerenal azotemia, the FeNa may be below 1%, suggesting avid tubular sodium reabsorption. FENa may be >1.0% in prerenal ARF if patients are on diuretics or with pre-existing CKD, salt-wasting syndromes or adrenal insufficiency. Ref: Harrison 19e 1807