Clinical Overview
The Osmolal Gap is an emergency medicine and toxicology tool. It compares the actual concentration of particles in the blood (measured in the lab via freezing point depression) against the theoretical concentration calculated from standard chemistry panels.
Pathophysiology & Evidence
Sodium, glucose, and blood urea nitrogen (BUN) account for almost all the osmoles in normal human serum. If a patient ingests a massive amount of an osmotically active substance—specifically toxic alcohols like methanol, ethylene glycol, or isopropanol—the measured osmolality will be significantly higher than the calculated value.
Formula Breakdown
Calculated Osmolality &= (2 * Na) \ &\quad + (Glucose / 18) \ &\quad + (BUN / 2.8) \ &\quad + (Ethanol / 4.6)
A normal osmolal gap is ≤ 10 mOsm/kg. A gap > 10 is highly suggestive of the presence of unmeasured exogenous osmoles.