Which is the correct formula for osmolality when concentrations are given in mg/dL?

• A. Osm = 2Na + Glucose/18 + BUN/2.8
• B. Osm = Na + Glucose + BUN
• C. Osm = 2Na + Glucose/18 + BUN/2.8 divided by 2
• D. Osm = Na^2 + Glucose^2 + BUN^2

Answer: (A)

Osmolality is calculated by summing the osmoles contributed by the major dissolved solutes in serum. Sodium contributes two osmoles per mole because each Na+ is accompanied by an anion, so its contribution is 2 × Na. Glucose and urea (BUN as a surrogate) contribute based on their molar concentrations, which require converting from mg/dL to mmol/L: glucose is divided by 18 (since 180 g/mol), and BUN is divided by 2.8 (approximate conversion from mg/dL to mmol/L for urea). Put together, the practical formula is Osm ≈ 2 × Na + Glucose/18 + BUN/2.8, yielding osmolality in mOsm/kg.

This is the best choice because it uses the correct sodium factor and the proper unit conversions for glucose and BUN. Options that sum the raw mg/dL values, or that include an extra division or square the terms, do not reflect how osmoles add up in solution.