What is the role of urine sodium measurement in the diagnosis of hyponatremia?

Updated: Dec 28, 2018
  • Author: Kartik Shah, MD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Answer

Urine sodium levels are helpful in distinguishing renal causes of hyponatremia from nonrenal causes.

Patients with hypovolemic hyponatremia due to nonrenal causes (eg, vomiting, diarrhea, fistulas, GI drainage, third spacing of fluids) have avid renal absorption of tubular sodium and urine sodium levels of less than 20 mEq/L, whereas those with hypovolemic hyponatremia due to renal causes (eg, diuretics, salt-losing nephropathy, aldosterone deficiency) have inappropriately elevated urine sodium levels in excess of 20 mEq/L.

Patients with hypervolemic hyponatremia due to decreases in effective circulating volume (eg, cirrhosis, nephrosis, congestive heart failure) have urine sodium levels of less than 20 mEq/L, whereas those with renal causes of hypervolemic hyponatremia or with SIADH have urine sodium levels in excess of 20 mEq/L.


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