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

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.

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.