Testing can be performed on a spot urine specimen. Specimens should be collected prior to starting therapy.
fractional excretion of sodium (FE-Na) in percent =
= (100% * (((urine sodium in mmol/L) / (plasma sodium in mmol/L)) / ((urine creatinine in mg/dL) / (plasma creatinine in mg/dL)))) =
= 100% ((urine sodium in mmol/L) * (plasma creatinine in mg/dL)) / ((plasma sodium in mmol/L) * (urine creatinine in mg/dL))
Urinary sodium levels between 20-40 mEq/L may be found in all forms of acute renal failure.
FE-Na < 1% may be seen in:
(1) 10% of cases of nonoliguric acute tubular necrosis
(2) some cases of acute tubular necrosis due to x-ray contrast material or myoglobinuria
(3) pre-renal azotemia
(4) acute glomerulonephritis
(5) early acute urinary tract obstruction
(6) early sepsis
FE-Na > 1% may be seen in:
(1) most cases of acute tubular necrosis
(2) after diuretic administration
(3) pre-existing chronic renal failure
(4) diuresis due to mannitol, glycosuria, bicarbonaturia
