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    Patent Pending

    Serum Osmolality/Osmolarity

    Calculates expected serum osmolarity, for comparison to measured osmolarity to detect unmeasured compounds in the serum.
    When to Use

    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.

    Note that at extremes of serum glucose, such as in hyperosmolar hyperglycemic state (HHS), osmolar gap is skewed, and clinical judgment should be used to make decisions regarding correction.



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    Creator Insights


    Methanol and ethylene glycol ingestions are treated by blocking alcohol dehydrogenase with fomepizol or ethanol, and often sodium bicarb if severely acidotic.


    Traditional equation:

    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.

    Dr. Roy Purssell

    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

    About the Creator
    Dr. Roy Purssell