Measured osmolality minus calculated osmolality is called what?

• A. Osmolal gap
• B. Osmolar gap
• C. Tonicity difference
• D. Freezing point depression

Answer: (A)

The difference between the measured osmolality and the calculated osmolality is the osmolal gap. This gap shows the amount of unmeasured osmotically active substances in the blood.

Measured osmolality reflects all osmotically active particles present, including electrolytes, glucose, urea, and possible substances like alcohols. Calculated osmolality estimates concentration from major contributors, commonly using 2 × [Na+] + [glucose]/18 + [BUN]/2.8, with glucose and BUN in mg/dL. Substances not included in that calculation—such as ethanol, methanol, ethylene glycol, isopropanol, or acetone—raise the measured value without changing the calculated one, producing a positive osmolal gap.

Normal osmolal gaps are typically around 0–10 mOsm/kg. A larger gap suggests an unmeasured osmolyte is present and can guide further testing. While some use the term osmolar gap, it’s more precise to refer to this as the osmolal gap because it relies on osmolality (mOsm/kg) rather than osmolarity. The other options don’t describe this difference between measured and calculated osmolality: tonicity difference relates to osmotic pressure across membranes, and freezing point depression is a method used to determine osmolality, not the gap itself.