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Serum Osmolality & Osmol Gap
Hematology / Fluid-Electrolytes
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.
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
Effective Osm = 2×Na + Glucose/18
Osmol Gap = Measured − Calculated (Normal: <10)
Ethanol contribution = Ethanol (mg/dL) / 4.6
Reference
| Parameter | Normal Range |
|---|---|
| Calculated Osmolality | 275–295 mOsm/kg |
| Osmol Gap | <10 mOsm/kg |
| Effective Osmolality | 285–295 mOsm/kg |
Clinical Notes
MEDUSA mnemonic for elevated osmol gap:
Methanol • Ethanol • Diuretics/DKA • Uremia • Sorbitol/Starvation • Acetone/Alcohols (isopropanol, propylene glycol)
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.
Methanol • Ethanol • Diuretics/DKA • Uremia • Sorbitol/Starvation • Acetone/Alcohols (isopropanol, propylene glycol)
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.
