Serum Osmolal Gap Calculator

About this calculator

The osmolal gap is the difference between directly measured serum osmolality (via freezing-point depression or vapor-pressure osmometry in a lab) and the osmolality calculated from routine chemistry values, typically using the formula 2 × sodium + glucose divided by 18 + BUN divided by 2.8. In healthy individuals this gap is small, generally under 10 mOsm/kg.

Emergency physicians and toxicologists use an elevated osmolal gap as a screening clue for the presence of unmeasured, osmotically active substances in the blood — most notably toxic alcohols such as methanol, ethylene glycol, or isopropyl alcohol, as well as severe ethanol intoxication or, less commonly, mannitol infusion — since these compounds raise measured osmolality without being captured by the standard calculated formula.

This calculator takes a patient's measured osmolality along with sodium, glucose, and BUN to compute the calculated osmolality and the resulting gap.