A variety of formulas have been proposed to predict changes in serum sodium concentration. All are based on an experiment done over 50 years ago by Edelman, who derived a formula relating the plasma sodium concentration to isotopically measured body sodium, potassium, and water. Some of these formulas fail because they do not include urinary losses of electrolytes and water. Even those that include these essential variables are not accurate enough for clinical use because it is impractical to adjust calculations to rapid changes in urinary composition, and because the formulas do not account for changes in serum sodium caused by internal exchanges between soluble and bound sodium stores or shifts of water into or out of cells resulting from changes in intracellular organic osmolytes. Nephrologists should curb their enthusiasm for predictive formulas and rely instead on frequent measurements of the serum sodium when correcting hyponatremia and hypernatremia.
Keywords: hypernatremia; hypertonic saline; hyponatremia; organic osmolytes.