Which osmolality value would be expected from empirical calculation for a normal serum?

• A. 188
• B. 204
• C. 270
• D. 390

Answer: (C)

Osmolality is determined by the number of osmotically active particles in a solution, and for serum the standard estimate uses the formula Osm = 2 × [Na+] + glucose/18 + BUN/2.8. The sodium concentration is the main contributor, so doubling it accounts for its accompanying ions. With typical normal values—Na about 140 mEq/L, glucose around 90 mg/dL, and BUN about 14 mg/dL—the calculation yields roughly 290 mOsm/kg. Among the given options, 270 is the closest representation of a normal serum osmolality, since the normal range is about 275–295 mOsm/kg. The other numbers are outside this typical range, either indicating hypo- or hyperosmolar states.