Estimation of Alcohol by Osmolal Gap Assessment

Another method for estimating ethanol, and other alcohol concentrations, is based on the increase in serum osmolality following alcohol ingestion. One important caveat of this technique is that osmolality should be measured using a freezing point depression technique. Use of a vapor pressure depression osmometer will result in underestimation of the serum osmolality. This occurs because alcohol, being a volatile substance, leads to an increase in the vapor pressure above the solution; nonvolatile solutes should normally decrease the vapor pressure above the solution. Measurement of the difference between measured osmolality, determined by freezing point depression, and osmolality calculated using sodium, urea, and glucose measured in the same sample, can be used to estimate the alcohol content of the sample. Each milligram of ethanol, methanol, or isopropanol present in the sample leads to a specific increase in the serum osmolality. The relationship between alcohol concentration and the corresponding increase in serum osmolality is as follows:

1 mg/dL ethanol = 0.22mOsm/kg increase 1 mg/dL methanol = 0.34 mOsm/kg increase 1 mg/dL isopropanol = 0.17 mOsm/kg increase

Discrepancies in the ability of these relationships to predict the presence of ethanol may be due to the presence of another volatile. However, some investigators have found that this technique may result in overestimation of alcohol concentrations by up to 30% (24). Thus, although this technique may be useful for estimating the presence of volatiles, they should not be relied upon exclusively for assessing the concentration of the volatile that is present. The gap may also be increased because of the presence of nonalcohol compounds such as mannitol and low-molecular-weight hydrocarbons. Current recommendations indicate that very high osmolal gaps (>50mOsM/kg) should be further investigated to determine the presence of an alcohol or other agent responsible for the increase in measured osmolality (25).

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