What is the role of the fractional excretion of uric acid (FEUA) and of phosphate (FEP) in the workup of cerebral salt-wasting syndrome (CSWS) (renal salt wasting)?

Updated: May 13, 2020
  • Author: Sudha Garimella, MBBS; Chief Editor: Sasigarn A Bowden, MD  more...
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Fractional excretion of uric acid (FEUA) is defined as the percentage of urate filtered by glomeruli that is excreted in urine. It is calculated by dividing the product of (urinary uric acid [mg/mL] x serum creatinine [mg/mL]) by the product of (serum uric acid [mg/mL] x urinary creatinine [mg/mL]) and multiplying the result by 100%. Normal values are less than 10%.

Patients with either cerebral salt-wasting syndrome or SIADH can have hypouricemia and elevated FEUA. However, after correction of hyponatremia, hypouricemia and elevated FEUA may normalize in SIADH but persist in cerebral salt-wasting syndrome (renal salt wasting). [5, 6]


Fractional excretion of phosphate (FEP) should be determined when evaluating patients with hyponatremia and hypouricemia. Elevated FEP suggests cerebral salt-wasting syndrome as opposed to SIADH. [6]

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