Use of formulas for calculating serum osmolality

Osmolality can be measured or calculated. More than ten formulas have been proposed for calculating serum osmolality from chemical concentrations. The aim of this work was to compare results obtained whith some of these formulas, those calculated in the laboratory using a multiple regression model and measurements with an osmometer. Scattered results for natremia and chloremia, depending on the automate, led us to define formulas adapted for each measurement system in order to reduce the risk of error in the evaluation of the " osmolar gap ".