Which components are used to calculate calculated osmolality?

• A. Sodium, Glucose, BUN
• B. Sodium, Glucose, Calcium
• C. Sodium, Potassium, Chloride
• D. Sodium, BUN, Chloride

Answer: (A)

Calculated osmolality estimates the plasma’s osmotic particle concentration. The main driver is sodium, because most extracellular solutes are sodium salts, and we account for the accompanying anions by effectively doubling the sodium contribution. The other important contributors are glucose and urea (measured as BUN), since they also influence the osmotic load in serum. The typical calculation uses these components: 2 × sodium + glucose/18 + BUN/2.8, giving the osmolality in mOsm/kg. That’s why the correct set of components is sodium, glucose, and BUN. Calcium, potassium, and chloride aren’t part of this standard formula, though they play roles in other physiological contexts.