Osmol gap

The serum osmolal gap (osmol gap) is calculated as the difference between measured and calculated serum osmolality. An elevated osmol gap indicates the presence of unmeasured osmotically active substances in the blood, most commonly toxic alcohols such as methanol, ethylene glycol, or isopropanol. It is a critical calculation in the emergency evaluation of suspected poisoning, unexplained metabolic acidosis, or altered mental status. A normal osmol gap effectively rules out significant toxic alcohol ingestion. It is also used alongside the anion gap to narrow the differential diagnosis in metabolic acidosis.