CMS Practical Nursing (PN) Pediatrics Practice Test

Isotonic dehydration means water and electrolytes are lost in roughly equal amounts, so the serum sodium stays about normal. Because the ratio of water to salt is preserved, the blood’s sodium concentration does not swing toward high or low values. This is why the description of equal losses with serum sodium near normal fits isotonic (isonatremic) dehydration.

If sodium were high, that would point to hypernatremic dehydration, which happens when water loss outpaces electrolyte loss. If water loss occurred while electrolytes were preserved, serum sodium would also rise, not stay normal. If electrolytes were lost without any water loss, that wouldn’t reflect typical dehydration with volume depletion and would present differently.