Alcohol dissolves in an aqueous environment, and because the water content of serum (~98%) is greater than that of whole blood (~86%), serum ethanol results are always higher than those obtained with whole blood. The serum : whole blood ethanol ratio is 1.14 (1.09-1.18) and varies slightly with hematocrit (5). Although most states have enacted laws that define intoxication while driving a motor vehicle based on whole blood ethanol levels, some states do not specify the specimen type. Therefore, laboratories that perform alcohol analysis must report the type of sample required.

Whole blood specimens should be drawn in a grey-top (potassium oxalate/sodium fluoride) Vacutainer. Preferably, the venipuncture site should be cleaned with an alcohol-free disinfectant, such as aqueous benzalkonium chloride (zephiran) or povidone iodine. Samples must be well capped and preferably refrigerated to prevent loss of ethanol. No ethanol was lost from whole blood specimens stored at 0-3°C or at room temperature (22-29°C) for up to 14 days (6). For longer storage or for nonsterile postmortem specimens, sodium fluoride can be used as a preservative to prevent increases in ethanol concentration because of fermentation.

Measurement of blood alcohol concentration is important for law enforcement as it is the measure for the determination of driving under the influence. Examples of punishable blood alcohol concentration limits include 80mg/dL (0.08 gram percent) in the USA, UK, and Canada; 0.20 mg/g in Sweden; and 0.50 mg/g in most EU countries (7). However, these values have no relevance to clinical management because there is no consensus on the concentration that defines clinical intoxication.

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