Retrograde extrapolations are a somewhat controversial calculation performed in forensic alcohol cases (DUI cases). The goal is to predict what someone’s alcohol concentration would be earlier in time, given a known concentration later in time.
The reason it is controversial is that is heavily relies on assumptions, many of which are not in evidence. The person performing the calculations must rely on population based averages. Some factors that might not be known, but have an effect on the calculations are:
Volume of distribution: This is the fraction of someone’s body that is comprised of water, as opposed to fat and bone. Alcohol distributes itself throughout the water in the body. The more water in someone’s body, the less their alcohol concentration will be given the same amount of alcohol dosed.
Volume of distribution is composed of two different factors (Widmark constant and subject weight). Widmark is a scientist who performed a series of experiments to determine a constant that could be used to convert someone’s weight, into the volume of distribution available for alcohol to distribute into. Widmark found two different constants, based on gender. The difference primarily being in his test subjects, women had a higher percent body fat than the men. This over generalization is probably not true today, with the growing problem with obesity.
In order for the calculations to be valid, it relies on the subject being fully absorbed. That is to say that there is no alcohol in the stomach of the person both at the time of the known alcohol concentration, AND back to the earlier time of interest. The problem here is that absorption rates vary considerably for each individual and certain factors (stress, drink composition, food consumed, biological variability) can influence that absorption time. There is also no general absorption time among experts. In Arizona some of the more “state biased” experts testify fully absorption within a half hour (with no eating or drinking history in evidence), many use one hour, and a growing number of experts (both government and independent experts) believe two hours (with no eating or drinking history) is an appropriate number.
Studies have shown people not being fully absorbed around 4 hours. So there is a lot of idiosyncratic variability.
And finally there is no way to know the elimination rate of the individual. Elimination rate is how rapidly someone “eliminates” (metabolizes) the absorbed alcohol. One again experts have to use either averages, or ranges of values for this calculation. Generally in Arizona the “median” elimination rate used is 0.015 g EtOH/ 100 mL blood/ hour. An appropriate range that covers the majority of social drinkers is 0.01 – 0.03 %BAC/hour. Some really good experienced heavy drinkers can be ever faster than that, and some people with very poor “tolerance” for alcohol, might have a much slower elimination rate.
The end result is an approximation based on averages, which may or may not be an accurate representation of the particular subjects earlier alcohol concentration. On top of that, it also relies on the “known” alcohol concentration to be accurate to begin with.
Some states don’t even allow for this type of evidence, others allow it. The state of Arizona is one that does.
What does this mean to the average social drinker? Not much. These aren’t the types of calculations one wants to do at a bar while meetings with friends or out on a date. But if you are looking for something to give you a rough ballpark estimate of your BAC, that is easy to use on your iPhone or iPod Touch, try Alculator.
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