Which expression correctly calculates osmolality from measured sodium, glucose, and BUN?

• A. 2Na + (Glucose x 0.0555) + (BUN x 0.357)
• B. 2Na + (Glucose x 0.0555) - (BUN x 0.357)
• C. 2Na + (Glucose x 0.1) + (BUN x 0.357)
• D. 2Na + (BUN x 0.357) + (Glucose x 0.0555)

Answer: (A)

Osmolality is the total concentration of osmotically active particles in plasma. The major contributors are sodium (with its accompanying ions) and small solutes like glucose and urea (BUN). Because sodium is accompanied by anions, its contribution is doubled, giving 2 × [Na]. To combine these into a single osmolality value, convert glucose and BUN from mg/dL to their osmotic contributions. For glucose, dividing by 18 converts mg/dL to mmol/L (and for a non-dissociating solute, that equals mOsm/kg), which is the factor 0.0555. For BUN, dividing by 2.8 gives its mmol/L contribution, corresponding to 0.357. Putting it all together: Osmolality ≈ 2Na + Glucose × 0.0555 + BUN × 0.357. This is why the expression with those factors and plus signs is the correct form. Subtracting terms or using an incorrect glucose factor (like 0.1) would misrepresent the osmolality, and the order of addition doesn’t change the result since addition is commutative.