Osmolal gaps (difference between measured and expected) of >10 can be caused by toxic alcohols (ethanol, methanol, ethylene glycol, isopropyl alcohol*); mannitol; and lorazepam infusions (which contain propylene glycol).
*Isopropyl alcohol causes a large osmolar gap but does not cause a anion gap metabolic acidosis.
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Methanol and ethylene glycol ingestions are treated by blocking alcohol dehydrogenase with fomepizol or ethanol, and often sodium bicarb if severely acidotic.
Serum osmolality, US units = (2 × Na + (BUN / 2.8) + (glucose / 18) + (ethanol / 4.6)
Purssell et al equation:
Serum osmolality, US units = (2 × Na + (BUN / 2.8) + (glucose / 18) + (ethanol / 3.7)
Note: the range displayed in the calculator uses the equations above.
Osmolal gap equation:
Osmolal gap = measured serum osm - calculated serum osm
Note: the osmolal gap displayed in the calculator uses the Pursell et al equation.
Facts & Figures
If values are in SI units, these are already molar (mmmol/L for example), so these do not need to be divided by their molecular weight.
Original/Primary ReferencePurssell RA, Pudek M, Brubacher J, Abu-Laban RB.Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap.Ann Emerg Med 2001;38 (6): 653–9.doi:10.1067/mem.2001.119455. PMID 11719745.
ValidationSiervo M, Bunn D, Prado CM, Hooper L. Accuracy of prediction equations for serum osmolarity in frail older people with and without diabetes. The American Journal of Clinical Nutrition 2014;100(3):867-876. doi:10.3945/ajcn.114.086769.
About the Creator
Roy Purssell, MD, is an emergency doctor at Vancouver General Hospital and a professor in the Department of Emergency Medicine at the University of British Columbia. He is also the medical director of the BC Drug and Poison Information Centre and the president of the Canadian Association of Poison Control Centres. Dr. Purssell's areas of research include impaired driving, substance abuse, laboratory testing and adverse drug reactions.
To view Dr. Roy Purssell's publications, visit PubMed