What is the role of serum osmolality in the diagnosis of ethylene glycol (EG) toxicity?

Updated: Dec 05, 2017
  • Author: Daniel C Keyes, MD, MPH; Chief Editor: Sage W Wiener, MD  more...
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Because ethylene glycol concentrations are not reported in a clinically helpful or timely fashion in most institutions, ethylene glycol exposure level is often estimated through measurement of the serum osmolality. This estimate is obtained by sampling a set of electrolytes and other serum solutes (eg, sodium, blood urea nitrogen [BUN], creatinine, glucose) and calculating the expected osmolality in the patient's serum. A serum osmolality is then measured, and the difference between the measured and calculated osmolality (the osmolal gap) is determined.

Several formulas are effective for calculating the osmolality from serum electrolytes and other solutes. The most commonly used formula in the US is 2(Na+ level) + BUN level/2.8 + glucose level/18 = calculated osmolality. The sodium level is measured in mEq, and the BUN and glucose levels are measured in mg/dL.

The osmolal gap is determined by subtracting the calculated osmolality from the measured osmolality (osmol [measured] – osmol [calculated] = osmolal gap). The serum osmolality must be determined by freezing-point depression rather than by boiling point elevation. This is because, with the boiling technique, the toxic alcohols are vaporized rapidly, and, thus, a falsely low or normal estimate of the osmolality is obtained.

A normal osmolar gap can range from -14 to +10, and potentially toxic ethylene glycol ingestions can be hidden within an apparently normal osmolar gap. Additionally, as described above, the osmolar gap goes away as the ethylene glycol is metabolized. For these reasons, the osmolar gap should never be used to exclude ethylene glycol poisoning. Additionally, moderately elevated osmolar gaps (10-30) are common in sick patients with disease states unrelated to toxic alcohol poisoning, so osmolar gaps in this range are not necessarily indicative of poisoning. However, largely elevated osmolar gaps (greater than 40) are highly suggestive of toxic alcohol poisoning.

Serum electrolyte levels are also useful later in the course of intoxication because they can reveal the presence of anion gap acidosis. This information may be important when determining the need for dialysis and other interventions. The goal of therapy, however, is to treat the patient before acidosis develops.

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