The serum osmolality is calculated by the concentrations of the different effective osmoles which compose it. The formula:
Calculated Sosm = (2 x serum [Na]) + [glucose] + [urea] (SI)
Calculated Sosm = (2 x serum [Na]) + [glucose, in mg/dL]/18 + [blood urea nitrogen, in mg/dL] (US)
WHY SODIUM TIMES TWO? The serum sodium is multiplied by two in order to account for the negatively charged solutes that must accompany it in the serum to maintain net neutrality in the body (i.e chloride and bicarbonate).
REFERENCES
- Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed, McGraw-Hill, New York 2001.