Figure 1. Characteristics of the blood alcohol concentration (BAC) curve Time for full absorption ranges from less than 30 minutes to more than three hours depending on presence or absence of food in stomach; type of beverage; caloric and alcoholic content of beverage; use of mixers; rate of consumption; use of drugs or medicines; physical and mental state of drinker; and individual characteristics of the drinker
ly by default, have become part of the conventional wisdom about the significance of BAC tests. Clearly, it is essential to find out how reliable the BAC test, upon which the court will rely, really is. To do so, we must examine how the BAC comes into being, how it rises and falls, and how it is eliminated (Figure 1). The rise and fall of the BAC
It is imperative that BAC results be both reliable and interprétable. Accurate and precise methods are available for the measurement of alcohol in blood and breath samples. Commercial instruments of considerable sophistication have been developed for the purpose, based on gas chromatography, IR absorption, spectrophotometric measurement of chemical and enzymatic reactions, and electrochemical fuel cells. The concentrations to be measured are low, bordering on trace analysis, yet many measurements are made by unskilled technicians with limited understanding of the principles involved. The alcohol concentration in blood is usually expressed as percent w/v, i.e., the grams of ethanol per 100 mL of blood, multiplied by 100. Most samples fall in the range of 0.04% to 0.25%. The methods used are usually capable of reproducibility within about ±0.01%, with well-maintained equipment and competent operators. Unfortunately, serious problems are involved both in obtaining valid samples and interpreting the analytical results. The human body is an exceedingly
complex, inhomogeneous dynamic system, and no two bodies are exactly alike. An individual's BAC is constantly changing, and the BAC of interest is not that at the time the sample is collected, but rather the value at the time of accident or arrest. To understand the nature of the BAC as a function of time, we must consider what happens in the body when alcohol is consumed. Alcohol is absorbed very slowly while in the stomach, but once it passes into the small intestine, it is rapidly absorbed into the bloodstream and carried to all parts of the body (Figure 2). If food is in the stomach, much of the alcohol is retained with it until the food is digested and moves on to the small intestine. Rates of digestion and stomach emptying vary greatly with individuals and with the kinds and amounts of food consumed. The rate of emptying of ordinary mixed meals is greatly influenced by fat content. A heavy meal with a high fat content may take four to six hours to be digested, with a corresponding delay in alcohol absorption. Alcohol on an empty stomach, however, tends to be absorbed completely in 20 to 60 minutes. The blood carries alcohol to all the organs and tissues of the body, where it is distributed in amounts proportional to their water content. Thus nerves, brain, and muscle achieve relatively high concentrations, whereas bone and fatty tissues absorb comparatively little. Alcohol is eliminated from the body mainly by enzymatic oxidation in the liver. A representative curve of BAC vs. time following rapid ingestion of a beverage such as gin or vodka on an empty stomach is shown in Figure 3, curve A. There is a rapid initial rise in the BAC; the rate of absorption decays exponentially with a half-life, in this instance, of about five minutes. If there were no elimination, the final BAC in this example would be 0.10%, but the liver is removing alcohol at a rate that lowers the BAC 0.015% per hour—a typical value. The resulting curve shows rapid increase, leveling off at about four half-lives followed by an increasingly linear decline.
Figure 2. Absorption and distribution of alcohol in the human body
slope of the straight line portion he could determine the rate of elimination, which he called β. Extrapolation of the linear portion of the curve back to time zero gave the blood alcohol concentration that theoretically would have been reached if all the alcohol were absorbed and equilibrated in stantaneously. From the body weight of the subject and the weight of alcohol consumed, Widmark could calculate the theoreti-
Widmark hypothesis
Widmark (4), in a 1932 study involving only 30 people, developed a method for estimating from a single BAC measurement both the amount of alcohol consumed and the BAC at times prior to the test sample. His subjects were given a known amount of alcohol per kilogram of body weight, as a single dose on an empty stomach. He then took frequent blood samples to plot curves of the type shown in Figure 3, curve A. From the
878 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
Figure 3. Theoretical curves of BAC against time Curve A, alcohol on an empty stomach; curve B, the same amount of alcohol with food
(continued
on p. 882 A)
