Laboratory Studies
Patients who ingest ethylene glycol may initially have few, if any, metabolic disturbances. Serum concentrations of ethylene glycol may be measured; however, at most health care facilities, these results are not available for 2 or more days. Thus, ethylene glycol concentrations are often not determined early enough to be useful in emergency treatment, though they should still be sent to confirm the diagnosis.
For institutions that frequently treat ethylene glycol toxicity cases, in-hospital rapid laboratory confirmation may become cost-effective because of the institutional cost-benefit ratio evaluation that compares therapy with fomepizole, ethanol, and hemodialysis. Emergency departments located in larger metropolitan areas may negotiate availability of this test at regional clinical laboratories. It is important to check on this availability at your own clinical site.
The classic laboratory profile of ethylene glycol ingestion is an early osmolar gap (the ethylene glycol serves as an unmeasured osmole) that disappears as an anion gap metabolic acidosis develops (as the ethylene glycol is converted into its acidic derivatives). [7] However, there is a wide range of normal osmolar gaps, and even patients with early presentations after consequential ethylene glycol ingestions may have a normal osmolar gap, so it should never be used to exclude toxicity. [8] Listed below are laboratory tests that will be useful in the setting of ethylene glycol ingestion.
Serum osmolality
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. See the Osmolal Gap calculator.
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.
Additional laboratory tests
The following tests should also be obtained in symptomatic patients:
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Serum calcium level
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Arterial blood gases
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Urinalysis: Urine may reveal the presence of calcium oxalate crystals, a sign usually observed late in the process of intoxication. Oxalate crystals typically have the shape of a folded envelope. (See the image below.)
Oxalate crystals. Courtesy of John D Schaldenbrand, MD, Department of Pathology, St Joseph Mercy Health System, Ann Arbor, MI.
Another technique, popularized by the television series "ER," is to shine a Wood's lamp (ultraviolet light) on an early sample of urine. If a sufficient fluorescein level is present in the radiator fluid, the urine fluoresces. [9, 10] The urine should be compared with a control sample. If the radiator fluid contains fluorescein, a green-colored glow may be observed in a dark room. This light also may be used to detect possible ethylene glycol on clothing or the patient. It must be stated, however, that this is not a reliable means to confirm or eliminate the possibility of an ethylene glycol ingestion, for a multitude of reasons. [11]
Imaging Studies
Imaging rarely contributes to the specific diagnosis of ethylene glycol intoxication, although in severe poisoning, computed tomography or magnetic resonance imaging scans of the brain may demonstrate thalamic lesions. Imaging may be useful as needed for routine care of these patients.
Other Tests
Other tests that may be useful depending on the clinical status of the patient include electrocardiography (ECG). Use of a Wood's (ultraviolet) lamp has been discussed above. Urine microscopy may be useful for identifying calcium oxalate crystals, as noted above. However, calcium oxalate crystals do not develop in the urine for about 4-8 hours following ingestion and if significant kidney insufficiency develops, they may not be present for 40 hours following the ingestion. [12]
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Oxalate crystals. Courtesy of John D Schaldenbrand, MD, Department of Pathology, St Joseph Mercy Health System, Ann Arbor, MI.
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Ethylene glycol.
