Tonicity balance, and not electrolyte-free water calculations, more accurately guides therapy for acute changes in natremia

The usual way to decide why hyponatremia or hypernatremia has developed and to plan goals for its therapy is to analyze events in electrolyte-free wat er (EFW) terms. We shall demonstrate that an EFW balance does not supply th is information. Rather, one must calculate mass balances for water and sodi um plus potassium separately (a tonicity balance) to understand the basis f or the change in natremia and the proper goals for its therapy. These point s are illustrated with a clinical example.