What is the role of the osmolar gap in the diagnosis of alcohol toxicity?

Updated: Jan 05, 2021
  • Author: Michael D Levine, MD; Chief Editor: Jeter (Jay) Pritchard Taylor, III, MD  more...
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Measuring the osmolar gap is important when toxic alcohols ingestion is suspected. The osmolar gap is determined by subtracting the calculated osmolality from the measured osmolality. The serum osmolality should be determined by freezing point depression rather than by heat of vaporization.

The serum osmolality can be calculated by the following formula:

Osm = (2) (Na+) + BUN/2.8 + Glucose/18 + EtOH/4.6 + Isopropanol/6.0 + MeOH/3.2 + Ethylene glycol/6.2

In the above formula, if, for example, an ingestion of methanol is suspected, the osmolality should be calculated using the sodium, BUN, and glucose. The ethanol level is also measured and then factored into the equation. If isopropanol and ethylene glycol are not suspected, they can be eliminated from the equation. Then, once the osmolar gap is determined, multiply the osmolar gap by 3.2 to determine the estimated methanol level.

It is important to recognize that neither the presence nor absence of an osmolar gap can be used to confirm or exclude a toxic alcohol ingestion. With both methanol and ethylene glycol, the alcohols are metabolized from an alcohol to an aldehyde, and ultimately to an acid. As such, shortly after an ingestion, the patient may have an osmolar gap without an anion gap. Similarly, in the later stages of an ingestion, a patient may have an anion gap without an osmolar gap.

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