How are urine studies used to differentiate SIADH from 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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Urinary sodium concentrations are typically elevated in SIADH and in cerebral salt-wasting syndrome (>40 mEq/L). However, urinary sodium excretion (urinary sodium concentration [mEq/L] x urinary volume [L/24 h]) is substantially higher than sodium intake in cerebral salt-wasting syndrome but generally equals sodium intake in SIADH. Therefore, net sodium balance (intake minus output) is negative in cerebral salt-wasting syndrome.

Urinary sodium excretion and urinary volume

A retrospective study by Arieff et al indicated that in patients with cerebral lesions who exhibit hyponatremia, urinary sodium excretion and urinary volume can be used to differentiate cerebral salt-wasting syndrome from SIADH. In patients with cerebral salt-wasting syndrome, these values were 394 ± 369 mmol/24 h and 2603 ± 996 mL/24 h, respectively. In comparison, the values were significantly lower—51 ± 25 mmol/24 h and 745 ± 298 mL/24 h, respectively—in patients with SIADH. [16]

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