Alcohol Calculations - The New Paradigms
The New 2024 AAFS Guidelines
Counterpoint Volume 8; Issue 3 - Article 2 (October 2024)
An article in the Upcoming Advanced Skills I Course
Jan Semenoff, BA, EMA
Forensic Criminalist
Article information:Article - 2300 words (approximately 12-14 minutes)
Companion Video - Counterpoint, Volume 8; Volume 2 - Article 3. CLICK HERE |
In 2024 the American Academy of Forensic Sciences established new guidelines regarding alcohol calculations that are to be used in legal proceedings. In a departure from traditional methodologies, the new guidelines fundamentally change several key calculations that have long been accepted by the courts.
It will be important for you to understand how these changes will impact testimony you receive in court. Traditional retrograde alcohol calculations have been replaced, as well as the starting point for these alcohol calculations.
What implications will these changes bring about? How will it impact your case?
Standard Drinks
One of the key changes in the new guidelines is establishing a national standard defining the notion of a Standard Drink. Although these values have been accepted in forensic circles for many years, I have noticed that the volumes and concentrations in so-called Standard Drinks vary from jurisdiction to jurisdiction. As a result of these new guidelines, Standard Drinks are now quantified as follows:
- Wines - 5 oz (150 milliliters) at 12% ABV
- Beers (ales, beers and ciders) - 12 ounces (355 milliliters) at 5% ABV
- Spirit alcohols (Gin, Vodka, Scotch, Bourbon, etc.) - 1.5 oz (45 milliliters) at 40% ABV
You should watch for these volumes carefully in court testimony, as the forensic toxicologist may be using local and therefore non-standardized notional volumes for these beverages. As a result, the interpretation of how many Standard Drinks were consumed may be affected by using non-standard volumes. Of course, varying the number of Standard Drinks will result in a different estimated BAC value in the alcohol calculations.
Changes to the Pharmacokinetics of Alcohol
The traditional Widmark’s Equation has been replaced by a series of new alcohol calculations. The new guidelines make some interesting assertions regarding the ADME (Absorption, Distribution, Metabolism, and Elimination) of alcohol through the human body.
Absorption
It is long been known that absorption rates are highly variable and are not necessarily linear in manner. Factors such as the presence of food in the stomach, and the type and volume of beverage consumed, along with the condition of the gastrointestinal tract can and does impact absorption rates. These variable factors are implicitly recognized in the guidelines.
But regardless of this recognition, under the new AAFS guidelines, all subjects are deemed to be post-absorptive for the purposes of alcohol calculations 2 hours (120 minutes) after they have consumed their last drink. This is in sharp contrast to previous studies that list the time to peak blood alcohol concentration as high as 166 minutes among normal healthy male individuals.
Remember that many factors affect absorption rates. In two separate studies, Dr. Kurt Dubowski found a wide range of times to peak BAC, anywhere from 9 to 166 minutes in his test subjects. See Figure 1.
Notice here that the second group (b) was given twice the dosage of alcohol of group a, and it resulted overall in a longer time to peak BAC. Also, notice the range of the peak values the test subjects obtained given an equivalent dose based on their body weight. The subjects should all ideally have a calculated BACs fairly close to the same, yet the measured range could be as much as 60% more than the BAC expected.
Peak BAC
The guidelines assert that a person has reached their Peak BAC two hours after a person consumes their last drink. They also identify that the time needed to reach the peak alcohol concentration (Peak BAC) is NOT the same as the time to reach the post absorptive phase, which occurs shortly thereafter.
Elimination
Following this high point or Peak BAC, the metabolic process of elimination occurs faster than any residual absorption, and the Elimination Phase begins, marked by a decrease of the BAC over time. What is important to note is that it is only during the post-peak, or elimination phase, that the ethanol in the body is truly at equilibrium. Why is this important?
It has been shown in the past that during the absorption phase of ADME, the reported breath alcohol result can over represent the true blood alcohol concentration. This is primarily since the alcohol concentration in the arteries is higher than the alcohol concentration in the veins during the beginning phase of absorption. Once equilibrium occurs, the alcohol concentrations between artery and vein are more closely aligned.
During the elimination phase, ethanol most closely follows a Zero Order kinetic in humans. Dr. A.W. Jones wrote in 2010 that the alcohol elimination followed a Zero Order kinetic until the subject had a BAC of about 0.020 grams per decilitre, at which time it eliminated following a First Order kinetic model.
As a result, with differences in absorption and elimination profiles any scenario is possible. Understanding the BAC profile is dependent upon having a complete and accurate picture of the precise drinking pattern, food consumption pattern, the medical history of the subject, and an understanding of their individual response to various factors, among a wide variety of variables. Given the uncertainty associated with the consumption pattern, any blood or breath alcohol readings obtained, however far apart, should only be considered representative of the BAC at the time the readings were obtained. See Figure 2.
However, under the new guidelines, the BAC obtained from a breath or blood test can be used as the starting point to calculate a blood alcohol concentration at a previous point in time.
Retrograde Extrapolation
In essence, Retrograde Extrapolations are no longer performed using the traditional Widmark Formula from circa 1930. See Figure 3:
Typically, we want to solve the equation for an alcohol concentration at a specific point in time, and so we re-arrange the formula to solve for Ct, as shown in Figures 4 & 4a:
Under the new AAFS guidelines, the formula is modified, as shown in Figures 5 & 5a:
The formula is essentially the same (the variables are simply re-named in the new equation), with the elimination of alcohol carried out as a separate calculation. For ease of comparison, here are Figures 4a and 5a again:
Widmark’s Rho and Volume of Distribution
Additionally, the new alcohol calculation is adjusted for what Widmark referred to as his Rho factor, indicated by the value r in the original formula. The Rho factor can be seen as a correlation between total body water in an individual, where the ethanol can be completely dissolved, and fatty tissues, where ethanol cannot be dissolved. Numerous factors impact an individual’s Rho value including their sex, body mass index, and age. In general, the Rho value is typically lower for women, obese individuals, and the elderly.
Rather than use Rho factors, the new calculations introduce the concept of Volume of Distribution (Vd). Due to the high variability within the population, the use of a single Volume of Distribution is inappropriate. Research supports a Vd range of:
- 0.58 to 0.83 L/Kg (liters per kilogram) for males and
- 0.43 to 0.73 L/Kg for females
These values are thought to represent a 95% range for each data set. Considering Widmark’s original Rho values were based on data from the 1930s and reflect the overall Body Mass Index of those individuals, perhaps a new Volume of Distribution more appropriately reflects body types and shapes found today.
Alternatively, an individual’s Vd may be estimated using anthropomorphic calculations when the subject’s sex, weight, age, and height are known. Several calculations are included in the guidelines to determine the individual’s Vd.
What is interesting is that the AAFS guidelines and Widmark’s original Rho values are not that far apart:
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Changing Best Practice
What is now allowed as best practice is to use a starting point from a breath or blood alcohol reading and extrapolate back from that reported amount to determine a BAC level at a previous point in time, referred to as Equation 9 in the guidelines, shown in Figure 6:
This new alcohol calculation merely adds an amount of alcohol to the reported reading based upon its likely elimination over time. It can be based on a measured alcohol concentration, whether from a breath sample or a blood draw, with a likely range of elimination added.
In essence the calculation is done twice. An elimination rate of 0.01 g/dL per hour is used for one calculation, and a value of 0.025 g/dL per hour is used for a second calculation.
As a result of these calculations, a breath or blood sample with a value of 0.31 g/dL obtained two-hours after an accident could well result in a calculated alcohol concentration range of 0.51g - 0.81g/dL. Adding 0.050 grams/dL to a reported reading is highly significant. See Figure 7:
Note the implication of these numbers. One places the accused motorist under the legal limit, and the other just above.
Compounding Error?
I remain concerned that using the results of a breath alcohol test as a starting point for these calculations adds the potential of error into the equation. It is well established that not all breath alcohol results (BrAC) are a true representation of an actual blood alcohol concentration (BAC). Indeed, the results may be falsely elevated due to a variety of issues. Using these reported results adds the potential for error upon error, and in my assessment, does not adequately reflect what best practices should be for these sorts of calculations.
Were they ever really performing Widmark calculations?
I have observed forensic experts giving testimony in court where a variety of variables were presented to them while on the stand (another practice the AAFS guidelines admonish). They fervently punch away into a calculator apparently doing some sort of alcohol determination when, in fact, the purported calculations are the result of precisely this practice - adding an estimated range of elimination to the previously reported result. I've always taken the position that this is a Substandard Practice and should not be performed. As a result, changing the guidelines to allow for these sorts of calculations raises the possibility that a judge or jury may consider calculated alcohol results that are highly suspect in nature.
I remain concerned that using the results of a breath alcohol test as a starting point for these calculations adds the potential of error into the equation. It is well established that not all breath alcohol results (BrAC) are a true representation of an actual blood alcohol concentration (BAC). Indeed, the results may be falsely elevated due to a variety of issues. Using these reported results adds the potential for error upon error, and in my assessment, does not adequately reflect what best practices should be for these sorts of calculations.
Were they ever really performing Widmark calculations?
I have observed forensic experts giving testimony in court where a variety of variables were presented to them while on the stand (another practice the AAFS guidelines admonish). They fervently punch away into a calculator apparently doing some sort of alcohol determination when, in fact, the purported calculations are the result of precisely this practice - adding an estimated range of elimination to the previously reported result. I've always taken the position that this is a Substandard Practice and should not be performed. As a result, changing the guidelines to allow for these sorts of calculations raises the possibility that a judge or jury may consider calculated alcohol results that are highly suspect in nature.
The guidelines implement some caveats in these calculations:
- First, these alcohol calculations must NOT be performed using urinalysis results as the starting point – blood or breath results ONLY.
- The guidelines explicitly state that a range of values must be used rather than an exact elimination rate. The elimination rate of 0.01 - 0.025 grams/dL per hour are used.
- The calculations must NOT use an estimated elimination rate based upon two or more readings obtained.
- The guidelines suggest that the calculations NOT be performed while giving testimony. The values need to be thoughtfully considered, with a range of assumptions and possibilities clearly outlined. They should be double-checked, preferably by another person, before being presented as expert opinion testimony.
- Additionally, the alcohol calculation shall NOT be performed under any circumstances with BAC levels lower than 0.020 g/dL as the starting point. Remember, the elimination may not be linear at low alcohol concentrations (below 0.020 g/dL).
Finally, and as previously discussed, if the time of the incident is more than two hours after drinking stops, the guidelines say that it is reasonable to assume that the subject is in the post-absorptive phase, regardless of the medical history of the subject.
If the drinking history is unknown, it is NOT reasonable to assume that the subject is post- absorptive. The guidelines state that “additional calculations should be applied to assess the impact of potentially unabsorbed alcohol”. Examples of the calculations are demonstrated in the guidelines.
Not to be outdone, the IACT training materials included with the guidelines clearly state that an elimination rate of 0.01 - 0.03grams/dL per hour can be used if you “want to cover more people?”. See below, left:
Additionally, in the Q&A section of the IACT training materials, the possibility of an extreme elimination rate is raised. The guidelines suggest that if you have information that "the individual is a chronic drinker", you may adjust the upper end of the elimination rate you use. See above, right. (Both screenshots from: Heartsill, Chris, IACT 2024 Workshop #2 Alcohol Elimination).
As a result, an elimination rate as high as 0.045 grams/dL per hour may be used without evidence to support alcohol abuse or chronic consumption patterns, at the sole discretion of the person performing the alcohol calculations. I would assume that "the information that the individual is a chronic drinker" would be adduced by a history of previous arrests or convictions for alcohol related offences and could therefore be opined with impunity.
Practice Tip: |
My concern in this regard is the variability in apparent metabolism rates for absorption time, time to Peak BAC, and metabolism or elimination rates. The literature for the last 50 to 60 years reports extensive values in this regard. However, I have observed that expert witnesses often commonly ascribe to one view or another and may not consider the full variability that exists in calculation assumptions.
As an example, some will say that full absorption will occur in 20 to 30 minutes, or that a Peak BAC will always be reached within 30 minutes following consumption, when the reported range is anywhere from 9 to 166 minutes, as we have discussed (Dubowski, 1985). Some will say that the elimination rate is fixed at 0.017g per hour, without taking the full range of elimination possibilities into consideration. When performing a retrograde extrapolation calculation, these values matter, and matter a great deal.
Other calculations
Additionally, the guidelines provide several other formulas to calculate the following:
- The minimum dose for a specific alcohol concentration
- The minimum number of standard drinks for a specific alcohol concentration
- The maximum alcohol concentration from a given dose
- The maximum alcohol concentration from a given number of drinks
Final thoughts:
I don't want to leave you with the impression that I am completely critical of most of the guidelines. I certainly have my concerns, and I've tried to articulate them here. In fact, many of the guidelines, and the new calculation formulas, present a balanced approach to the issue of alcohol calculations, particularly in a forensic context. While the guidelines do appear to present several issues that may need to be addressed in the future, they are, as of this point, a way forward by which the alcohol calculations can be performed. I would respectfully suggest we need to proceed with caution so that the integrity of the process is maintained.
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For further study:
- American Academy of Forensic Sciences, Academy Standards Board, Best Practice Recommendation for Performing Alcohol Calculations in Forensic Toxicology, 2024.
- Dubowski, K.M. Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects, 10 J. Stud. Alcohol Suppl. (1985).
- Best Practice Recommendations for Performing Alcohol Calculations in Forensic Toxicology – 2024 Update, 2024 IACT Workshop, April 2024.