Calculate estimated and measured osmolality with gap.
Calculated Osm = 2×Na + Glucose/18 + BUN/2.8. Normal: 275–295 mOsm/kg. Osmolal gap = Measured − Calculated. Normal gap ≤ 10. Elevated gap suggests unmeasured osmoles: ethanol, methanol, ethylene glycol, isopropanol. Each 100 mg/dL ethanol adds ~22 mOsm. The gap helps identify toxic alcohol ingestions.