Calculated serum osmolality estimates total osmotic activity from key solutes. The osmol gap — the difference between measured and calculated osmolality — detects unmeasured osmoles such as toxic alcohols. An elevated osmol gap (>10 mOsm/kg) warrants investigation.
Calculated Osmolality
Effective Osmolality
Osmol Gap:
Ethanol contribution to osmolality:
Formula
Calculated Osm = 2×Na + BUN/2.8 + Glucose/18 Effective Osm = 2×Na + Glucose/18 Osmol Gap = Measured − Calculated (Normal: <10) Ethanol contribution = Ethanol (mg/dL) / 4.6
BUN contributes to calculated but NOT effective osmolality because urea freely diffuses across cell membranes (ineffective osmole). An elevated effective osmolality indicates presence of impermeable solutes causing cellular dehydration.
For educational use only. Clinical decisions must incorporate full patient evaluation.
