The reporting of ethyl alcohol testing has been the source of much confusion over the years between individuals in the health care field, and those who use alcohol results for forensic purposes. In most states, the accepted limit of alcohol concentration is typically defined as 0.10 gram percent in whole blood. Some clinical laboratories measure alcohol concentrations in serum or plasma and report values in milligrams per 100 milliliters (mg/dL). Because ethanol is very water soluble, its distribution in various body fluids is dependent on the water content of those fluids (42). The water content for serum is typically 98% while for whole blood, the water content is about 86% (with a normal hematocrit). Therefore whole blood alcohol concentrations are lower than serum or plasma values. However, a constant conversion factor cannot be applied because the hematocrit can dramatically change from individual to individual. It should be noted that these legal definitions have little or no clinical meaning in the emergency department.

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Methyl alcohol and ethylene glycol are substances that are not toxic by themselves, but produce metabolites that can produce significant morbidity and mortality. Methyl alcohol metabolizes to formaldehyde and then to formic acid, which produces a significant metabolic acidosis (43). Ethylene glycol breaks down to oxalic and glycolic acids, both of which contribute to a significant metabolic acidosis (44). Ethylene glycol also contributes to significant renal tubular necrosis. Detection of these intoxicants in blood is important for therapeutic management. A new antidote, fomepizole (Antizol), a competitive inhibitor of alcohol dehydrogenase (enzyme responsible for methanol and ethylene glycol conversion), is FDA-approved for the treatment of ethylene glycol (45) and has been used in cases of poisoning by methanol (46).

In the assay of Vinet et al., methyl alcohol is converted to formaldehyde by alcohol oxidase (47):

                                 alcohol oxidase
   methanol + oxygen -------------------------à formaldehyde + hydrogen peroxide

                                          formaldehyde dehydrogenase
formaldehyde + H₂0 + NAD⁺ ----------------------------------------à formic acid + NADH

In the modified assay of Ochs et al., ethylene glycol reacts with glycerol dehydrogenase (48):

                                  glycerol dehydrogenase
ethylene glycol + NAD⁺ ------------------------------à hydroxy-acetaldehyde + NADH

This assay requires removal of the endogenous triglyceride concentration with the addition of lipase to the reagent. The presence of high concentration of l-lactate dehydrogenase and lactic acid interferes with this assay producing false positive results (49). A thin-layer chromatographic procedure for methanol and ethylene glycol in plasma has also been developed (50).

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The Committee recognizes that many laboratories may not have instrumentation where non-commercial reagents can be placed, or are not capable of preparing and validating home brew reagents. Surrogate markers, e.g., measurement of the serum osmolality and calculation of the osmolal gap have been studied as alternative to direct assays of these alcohols (51,52). The osmolality of a fluid is defined as the number of moles of solute dissolved in a kilogram of solvent. Osmolality is typically measured using the freezing point depression method, and has a serum reference range of 275-295 mOsm/kg. In normal serum, the major contributing components are the monovalent electrolytes, glucose, and blood urea nitrogen. The calculated osmolality is based on the contributions of these components, and the gap is the difference between measured and calculated.

Calculated osmolality = 2 [Na] + [glucose ¸ 18] + [BUN ¸ 2.8]

Osmolal gap = measured osmolality – calculated osmolality

Many formulas have been proposed for the calculation of osmolality (51,53). The one given above appears to produce a normal osmolal gap, i.e., close to zero, as any of the formulas. The gap is increased in the presence of methyl and ethyl alcohols, acetone, ethylene glycol, and higher glycol ethers, or salicylates, proportional to their millimolar concentrations in blood. Measurements for ethyl alcohol are readily available, and should be considered in cases where there is a significant increase in the gap (e.g., >10 mOsm/kg). The anion gap is the difference between measured cations (sodium) and anions (chloride and total CO₂. Normal anion gap is about 12 mmol/L. The gap is increased in patients with a metabolic acidosis due to a toxic ingestion of methanol and ethylene glycol, due to the acidosis and a low total CO₂ concentration.

Discussion

Although an increased gap in the presence of a metabolic acidosis suggests the presence of methyl alcohol or ethylene glycol, there are other conditions that are associated with an elevated osmolal gap. In a patient who has an established metabolic acidosis from toxic alcohol ingestion, a normal or falsely low osmolal gap can occur if blood is sampled after the volatile alcohols have been converted to the acid metabolites (51). Increases in the osmolal gap can occur in patients with multiple organ failure, and produce a false indication of volatile alcohol exposure, and other unmeasured osmolal entities (54,55).

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