What is the upper limit of the normal osmolal gap (mOsm/kg)?

• A. 8 mOsm/kg
• B. 10 mOsm/kg
• C. 12 mOsm/kg
• D. 15 mOsm/kg

Answer: (B)

Osmolal gap reflects unmeasured osmotic substances in the blood. Measured osmolality should roughly match the calculated osmolality, which is commonly estimated from 2Na plus glucose/18 plus BUN/2.8. In healthy individuals, the difference between measured and calculated values is small, typically up to about 10 mOsm/kg. When the gap is larger than this, it suggests the presence of unmeasured osmoles such as alcohols or other toxins. Because the normal upper limit is around 10 mOsm/kg, that value is the best answer.