Which formula is used to calculate the osmolar gap in suspected toxic alcohol ingestion?

• A. 2Na + BUN/2 + glucose/10
• B. 2Na + BUN/3 + glucose/20
• C. Na + BUN + Glucose
• D. 3Na + BUN/3 + glucose/20

Answer: (B)

Osmolar gap is used to detect unmeasured osmoles, such as toxic alcohols, by comparing the measured serum osmolality with an estimated osmolality calculated from the main serum solutes. The estimated osmolality is formed from sodium (which drives most of the plasma osmolality) and smaller contributions from glucose and BUN, converted from mg/dL into osmoles. Using two times the sodium concentration plus glucose divided by twenty plus BUN divided by three provides a simple, widely used approximation to convert those concentrations into mOsm/kg. This version matches the standard rounded formula: calculated osmolality ≈ 2 × Na + glucose/20 + BUN/3. The osmolar gap then equals measured osmolality minus this calculated value. In suspected toxic alcohol ingestion, the measured osmolality rises due to the alcohols themselves adding to the osmolar load without being included in the calculation, so the gap increases. Other formulas that omit the doubling of sodium or use different divisors would misestimate the calculated osmolality, making the gap appear smaller or larger than it truly is.