Serum Osmolality Normal — FRCA Primary MCQ
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Correct answer: E — 275–295 mOsm/kg, the usual adult serum osmolality range
The best answer is “275–295 mOsm/kg, the usual adult serum osmolality range”. Normal measured serum osmolality is approximately 275–295 mOsm/kg. It is determined mainly by sodium and associated anions, with glucose and urea contributing. In SI units, calculated osmolality is commonly estimated as 2 × sodium + glucose + urea, all in mmol/L; a raised osmolar gap suggests unmeasured solutes.
Reference: Royal College of Anaesthetists, Primary FRCA Syllabus 2021: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf