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
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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)
Figure 4. Plot of Widmark's r factor against β for his subjects Open circles, men; filled circles, women. Both r and β show wide scatter about their averages with no correlation between r and β
cal whole-body alcohol concentration (WBAC) at time zero. The ratio of WBAC to BAC he designated as "r." If an individual's BAC is measured at some point on the declining portion of the curve, and if his or her r value is known, then (BAC)(r) = WBAC in percent by weight. From the body weight and the total body alcohol, he estimated the weight of alcohol that "must" have been imbibed to give the observed BAC. To estimate the BAC at some prior time (but still after the peak), he applied a correction by add ing 0.015% (his average value of β) to the test BAC value for each hour elapsed between the time of interest and the taking of the blood sample. Thus he calculated the BAC an hour or two earlier during the period of steady decline. Widmark was very well aware of the limitations of his method and the great uncertainties in the assumptions that had to be made when applying it. Unfortunately, although his approach and principles are still the foundation for such calculations, the limitations and uncertainties he noted are gener ally ignored. In actual cases, the value of the individual's r is never measured and is not known. Instead, Widmark's population average, 0.68 for men, is assumed to apply to any chance sub ject. Even in Widmark's small group, the r values ranged from 0.47 to 0.86, and the average for women was signifi cantly lower than for men. Similarly, Widmark's average β (0.015%), or a value close to it, is applied to all sub jects, despite the fact that Widmark's /3's spread quite evenly over the range of 0.010% to 0.020% (Figure 4). Later
studies with larger populations con firmed his findings and showed results as low as 0.006% and as high as 0.040%. Numerous charts, tables, and calcu lating devices are in common use in many states. They are sometimes dis tributed by state officials or depart ments and are often used in alcohol education courses, and they purport to guide the drinking public as to the probable BACs to be expected after consumption of given amounts of alco hol. The calculations are based on the use of Widmark's average r for men (0.68). As a result, they are dangerous when used by women or low-r men. The "probable BACs" in question can in fact be reached on considerably less alcohol than is suggested by the charts when used by low-r persons. The possibilities for error when esti mating probable BACs, prior BACs, or alcohol amounts consumed are com pounded if Widmark's methods are applied in situations that do not corre spond to his "model situation"—hard liquor in a single dose on an empty stomach (5). The wide range of BAC values found when Widmark gave his subjects the same amount of alcohol per kilogram of body weight is seen in Figure 5. Studies show that when alco hol is taken with a meal the rise in BAC is slower, and lower peaks are generally obtained (Figure 3, curve B). A drinker may hold his BAC nearly constant for a period of hours when intake balances elimination. Roller coaster rises and falls in peaks occur over periods when drinking is inter mittent. Beer presents separate cal culation problems; it is less rapidly ab
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sorbed than hard liquor and gives low er peaks with the same amount of al cohol. Attempts to calculate r from a beer curve can give ridiculous values that suggest that the whole-body alco hol concentration is greater than the concentration in the blood. The timing of sample collection is extremely important. An individual who has had a number of drinks with a fairly heavy meal may not reach his or her peak BAC for one to three hours after the end of drinking. If, driving home after such a meal, one has an ac cident and is taken into custody, he is not likely to be tested for another 30 minutes to 2 hours after arrest. His BAC at the time of the accident may have been well below 0.10% but sig nificantly above that value by the time the test is taken. The prosecution automatically assumes that the BAC was falling, rather than rising, be tween the incident and test. Their ex perts will typically add 0.015% to the test value for each hour elapsed from arrest to test. Assume, for example, a slowly rising BAC that had reached 0.06% at incident time and a test BAC of 0.12% two hours later. Using the ap proach commonly employed by pros ecution experts, that driver would be "estimated" to have had a 0.15% timeof-incident BAC, an unconscionably unfair result by either scientific or le gal standards. Such complications do not imply that single chemical tests for intoxica tion are necessarily worthless. A prop erly determined "high" BAC can ef fectively refute the "two beer" de fense. One high value, together with other substantial evidence of intoxica-
Figure 5. Maximum BAC values result ing from the ingestion of the same weight of alcohol per kilogram of body weight in Widmark's subjects Open circles, men; filled circles, women
tion at the incident time, may conclu sively eliminate the suggestion of oth er causes for the conduct observed. Conversely, the finding of a low or negligible BAC on one test may excul pate the subject or may support a sug gestion of other causes for the ob served behavior. What is not proper, however, is the overinterpretation of a single data point to infer a specific pri or BAC or amount of alcohol con sumed. The significance of any BAC measurement must always be deter mined within the context of time as one point on a continuum and not as a static value. Although the procedure would not eliminate all interpretation problems, requiring a minimum of two tests, at least one half hour apart, would be of great value in assessing BAC results, and the time and ex pense would be minimal. Breath testing For forensic purposes, the conver sion of breath alcohol to BAC is statu torily required. Most tests offered in court are done on expired breath, rather than on blood, for many rea sons. It is a noninvasive technique, it is rapid, it can be used in the field by police officers rather than medical or laboratory technicians. Unfortunately, all breath-testing devices are designed to permit uniform use with chance in dividuals from a large, heterogeneous population, and so certain assump tions are made about the tested indi vidual that may or may not be true. To convert breath alcohol to BAC, a conversion factor of 2100:1 is used, on the assumption that 2100 mL of deep lung breath has the same weight of ethanol as 1 mL of blood when the two phases are in equilibrium at normal body temperature. In fact, this is true for only a relatively small portion of the population. The value of 2100:1 was the compromise achieved by a committee that examined the results of more than 25 studies in which aver ages ranging from 1142:1 to 3478:1 were reported, with individual values much higher and lower. This very wide range of values is un derstandable. Many significant varia bles in addition to experimental tech nique affect the result. Deep-lung (al veolar) air may or may not be in equilibrium with the blood. The sam ple of air taken from the mouth may or may not be representative of alveo lar air. Variable amounts of tidal air may be present depending on the sub ject's condition or on how long and hard the subject was persuaded to blow. The bronchial passages, throat, and mouth, through which the sample passes, all have moist surfaces that ex change alcohol with the breath as if it were a sample going through a chro matographic column. There is a sub-
stantial temperature gradient between mouth and lungs, and the temperature is in a range where the vapor pressure curve of ethanol is rising steeply. "Normal" body temperature is not a fixed value but covers a range of sever al degrees. Fever can elevate the alco hol content of the breath and the pres ence of drugs such as aspirin can lower it significantly. Other factors, such as the hematocrit, can also affect the re sult. Although the use of the factor 2100:1 to convert breath to BAC may give a fairly reasonable estimate for many people, there are a substantial number who may be falsely convicted or improperly exonerated by the re sult. Mason and Dubowski (6), who are recognized experts on breath anal ysis, say simply that the true value, if indeed there is such, lies somewhere in the range of 1900 to 2400:1. When breath and blood tests are performed simultaneously and the results com pared, even under ideal laboratory conditions with highly trained person nel, significant troubling differences in results are consistently recorded. Breath testing is, in any event, par ticularly vulnerable to contamination because of the low levels involved. The traditional Breathalyzer sample, 52 mL, contains only 25 μg of ethanol when the BAC is 0.10%. Any traces of alcohol trapped by dentures, any burp bringing alcohol vapors from the stomach, can have catastrophic ef fects. An individual whose BAC is zero, merely by taking a few apprecia tive sniffs over an open bottle of 50% alcohol, can send a breath tester off scale for several minutes afterward (7). Failure to save the breath sample for later independent testing effective ly eliminates any hope of detecting op erator error or mechanical malfunc tion, but very few states require that such samples be saved, although the technology is now available at minimal cost. Blood testing: Postaccident and postmortem samples The measurement of alcohol in a given blood sample is relatively straightforward, but methods of col lection and preservation can present problems, and correctly interpreting test results can be very difficult. Blood samples are commonly obtained in cases of serious injury or death. Sam ples obtained and tested by hospital
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personnel are not necessarily adequate for forensic purposes. The emergency room physician, observing an accident victim with an odor of alcoholic bever age on his breath, wants to determine as quickly as possible whether the symptoms he sees are the result of a high BAC or of injuries. For his pur poses, a result with an uncertainty of even ±0.04% may be more than ade quate. To offer that same result in evi dence as "the" BAC, however, may be entirely inappropriate. For forensic purposes, more careful procedures ought to be required for BAC measurements than might usual ly be used in routine clinical practice. Special attention must be given to documenting the chain of custody of the sample, to alcohol-free sample col lection, to the use of proper anticoagu lants and preservatives, and to adop tion of testing procedures that ensure a high degree of precision and reliabil ity (such as testing "knowns" before and after and running more than one test per sample). Protocols should be adopted for sample handling and test ing and for equipment calibrations, standardizations, and maintenance, and permanent records should be kept that document all phases of sample measurement and equipment perfor mance. Medical records must be carefully reviewed in accident and death cases. The actual injuries, and all medical treatment afforded before sample col lection, or before death, must be care fully reviewed before opinions are formed about the significance of a later-test BAC. Did anything disrup tive occur to raise questions about the true relationship of the test BAC to the actual, circulating BAC of that in dividual before the accident when all his normal processes and functions were intact? It has been common practice in death cases in many jurisdictions for coroners, hospital technicians, and some medical examiners to collect purported heart blood for BAC testing without an autopsy. This is done by needle puncture aimed through the chest wall at the heart. The sample collected may be "heart blood," or only a "bloody fluid" resulting from the injuries that caused death. When postmortem samples are submitted for BAC determinations, separate tests are not done to show the com plete composition of the sample, nor are procedures followed that would confirm that a normal, whole-blood sample is being tested. The results can be totally misleading. Obviously, the more extensive the trauma that has occurred, including rupture of organs and blood vessels, the more subject to gross error such "blood" sampling is. Contamination of the sample by al-
