Normal serum osmolarity is approximately:

• A. ~250 mOsm/L
• B. ~300 mOsm/L
• C. ~350 mOsm/L
• D. ~400 mOsm/L

Answer: (B)

Normal serum osmolality reflects the concentration of solutes that influence water movement in the extracellular space, and the main driver is sodium with its accompanying ions. With a typical serum sodium around 140 mEq/L, the osmotic contribution from sodium and its partners is about 280 mOsm/kg. Adding normal glucose (90 mg/dL, about 5 mOsm/kg) and urea/BUN (14 mg/dL, about 5 mOsm/kg) brings the total to roughly 290–300 mOsm/kg. Clinically, this centers near 300 mOsm/L, so that value best matches normal. Values much lower or higher would indicate abnormalities in sodium, glucose, or urea levels.