Monday, March 23, 2009

The osmolar gap in toxic alcohol poisoning

A paper in BMC Emergency Medicine reported a systematic evaluation of test characteristics of the osmolar gap to identify patients with levels of methanol and ethylene glycol considered toxic:

Conclusion: In this sample, an osmole gap threshold of 10 has a sensitivity and negative predictive value of 1 for identifying patients for whom hemodialysis is recommended, independent of the ethanol coefficient applied. In patients potentially requiring antidotal therapy, applying an ethanol coefficient of 1.25 resulted in a higher specificity and positive predictive value without compromising the sensitivity.

Caution: Although the test characteristics were good this study only looked at the ability of the osmolar gap to predict direct measurements of toxic alcohol levels, not clinical outcomes. Late in the course of toxic alcohol ingestion the parent alcohol becomes extensively metabolized to toxic acids which dissociate in the blood. After buffering of hydrogen ion the anionic species must be associated with sodium ion which enters into the osmolarity calculation. These acids, then, contribute to the anion gap but not the osmolar gap. The osmolar gap, which reflects the concentration of the parent compound, may be deceptively low late in the course of ingestion.

The best discussion of toxic alcohol ingestion I’ve seen is the section in UpToDate, which makes the above points and emphasizes that there’s no absolute cut off in the osmolar gap for treatment decisions.

In the body of the BMC paper there are some useful clinical points as well as equations for calculation the osmolar gap in patients with elevated ethanol levels, with and without a correction factor.

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