Notes & Sources

• Divisors convert mg/dL to mOsm/kg: glucose ÷ 18, BUN ÷ 2.8, ethanol ÷ 3.7 (molecular weight ÷ 10).
• The osmolar gap is measured minus calculated osmolality and requires a measured value by freezing-point depression. A gap above roughly 10 mOsm/kg suggests unmeasured osmoles.
• In suspected toxic alcohol ingestion, include ethanol in the calculation — otherwise ethanol alone produces a large gap and masks the question you are actually asking.
• A normal osmolar gap does not exclude toxic alcohol ingestion, particularly late in the course once the parent alcohol has been metabolised to its organic acid.