Which formula should be used to estimate osmolality when Na, Glucose, and BUN are expressed in mmol/L?

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Multiple Choice

Which formula should be used to estimate osmolality when Na, Glucose, and BUN are expressed in mmol/L?

Explanation:
Osmolality reflects the total number of osmotically active particles in solution. Sodium salts contribute about two particles per Na+ because Na is always accompanied by a counterion, while non-electrolytes like glucose and urea contribute one particle each per mole. When Na, glucose, and BUN are all given in mmol/L, the estimate is Osm ≈ 2×Na + Glucose + BUN. This aligns with how the sodium-containing salts add twice as many particles as Na alone, and how glucose and BUN each add one particle. For example, with Na 140 mmol/L, glucose 5 mmol/L, and BUN 7 mmol/L, Osm ≈ 2×140 + 5 + 7 = 292 mOsm/kg, which matches typical serum osmolality. Using Osm = Na + Glucose + BUN would underestimate osmolality significantly and is not appropriate when all values are in mmol/L.

Osmolality reflects the total number of osmotically active particles in solution. Sodium salts contribute about two particles per Na+ because Na is always accompanied by a counterion, while non-electrolytes like glucose and urea contribute one particle each per mole. When Na, glucose, and BUN are all given in mmol/L, the estimate is Osm ≈ 2×Na + Glucose + BUN. This aligns with how the sodium-containing salts add twice as many particles as Na alone, and how glucose and BUN each add one particle. For example, with Na 140 mmol/L, glucose 5 mmol/L, and BUN 7 mmol/L, Osm ≈ 2×140 + 5 + 7 = 292 mOsm/kg, which matches typical serum osmolality. Using Osm = Na + Glucose + BUN would underestimate osmolality significantly and is not appropriate when all values are in mmol/L.

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