Utility of urine and plasma osmolality in the evaluation of pediatric hypernatremia

In cases when the cause of pediatric hypernatremia remains uncertain, comparing the urine osmolality (Uosm) with the plasma osmolality (Posm) may be helpful in establishing the underlying etiology. Uosm that is less than Posm suggests that the child has a concentrating defect, which increases the likelihood of excess urinary water loss. In contrast, children with Uosm greater than Posm have an intact urinary concentrating mechanism and are more likely to have hypernatremia due to excess gastrointestinal or skin water loss, or excess salt intake.