What is the initial diagnostic approach to hypotonic hyponatremia?
Once hypotonicity is confirmed with an effective osmolality <275 mOsm/kg H 2 O, the urine osmolality is instrumental is assessing whether or not the kidneys are reabsorbing electrolyte-free water. In the absence of chronic kidney disease, a urine osmolality <100 mOsm/kg indicates an intact diluting mechanism and a state of maximal electrolyte-free water excretion. Conversely, a urine osmolality >100 mOsm/kg suggests that electrolyte-free water reabsorption is occurring, which would be inappropriate from the point of view of tonicity alone. Patients with chronic kidney disease may have an impairment in maximal ability to dilute the urine. In such cases a urine osmolality of <200 mOsm/kg reflects maximal free-water excretion.
