What is the normal serum osmolality range (in mOsm/kg)?

• A. 275-300
• B. 250-275
• C. 300-325
• D. 325-350

Answer: (A)

Normal serum osmolality reflects the concentration of osmotically active particles in plasma. In healthy people, this value is about 275 to 295 mOsm/kg, and many reference ranges round to 275–300 mOsm/kg. Therefore, the option spanning 275–300 mOsm/kg best fits the commonly accepted reference interval, capturing both typical lower and upper limits. The other ranges fall outside what is normally observed: 250–275 is generally below normal, while 300–325 and 325–350 are above normal. Osmolality is mainly determined by sodium with its accompanying anions, glucose, and urea, and it remains tightly regulated by water balance in the body.