Diagnostic algorithm of hyponatremia

Hyponatremia is the most common electrolyte disturbance and is associated with increased morbidity and mortality. It is driven by an excess of free water relative to total body sodium. While determining the underlying cause(s) of hyponatremia can be challenging, this can be facilitated by an algorithmic approach. Hypotonic hyponatremia is diagnosed by excluding translocational and pseudohyponatremia and confirmed by measuring plasma osmolality. Measuring urine osmolality and urine sodium concentration together with clinical history and examination, especially assessment of volume status, can determine the underlying cause. The most common cause of hyponatremia is the syndrome of inappropriate diuresis, characterised by inappropriate arginine vasopressin activity resulting a high urine osmolality and high urine sodium concentration. Further investigation can determine the underlying cause(s) of the syndrome of inappropriate antidiuresis. This review provides a diagnostic algorithm for hyponatremia, with a focus on biochemical parameters supplemented by clinical fluid status examination.