What is retrograde extrapolation and why should I be worried about it?

Imagine you are leaving the local Applebee's after having a cocktail or two. You wait a half hour and drink a water. You feel fine, so you start driving home. As you exit the parking lot and begin making a right turn on the street, the red and blue lights light up in your rear-view mirror.

The cop asks you to exit the vehicle because he can smell alcohol on you. Because you feel fine, you exit the vehicle and comply with his requests to do a number of test right there on the side of the road. Right after you finish, he tells you to turn around and put your hands behind your back.

He takes you down to the station, some more procedural things are done. An hour or so passes. Then he has you blow into the breath machine. The results come out .06. You think, "Great, I am well under the legal limit of .08." You are then placed into a jail cell where you get to spend the night. You do not make it into work the next morning, instead seeing the local magistrate to ask to be released from jail pending trial. You spend the next 3-5 months going taking days off of work to attend various court dates.

You get to trial and your defense is that you were under the legal limit of .08. The government then puts on a toxicologist who says, well the result at the time of the test was .06, but if we use retrograde extrapolation, your BAC level was higher an hour and a half ago before you were pulled over! In fact, you were over the legal limit, so you are guilty. A jury, trusting the government science and scientist, convicts you of DUI.

So, what is a retrograde extrapolation? Generally, in Washington the testimony will involve the application of Widmark's formula to a particular situation.

Widmark's formula has some variations, so the formula is not truly a set formula (which is a pretty big problem). One formulation of Widmark's is BAC = (o x 5.14 / w x r) - .015 x h. The variables would work out to "o" being ounces of alcohol consumed; "w" being weight of the person in pounds; "r" being a so-called gender constant (.73 for males and .66 for females), h would be hours since the drinking started. The .015 is the supposed average elimination rate for alcohol or the amount a perhaps BAC supposedly metabolizes per hour. To oversimplify Widmark's, they are basically adding .015 to your BAC per hour.

The first problem is that alcohol is absorbed in the body and then eliminated. For instance, if you took three shots of alcohol back to back. You would not feel the effects intoxication until the alcohol actually entered your blood stream, which would take a half hour or longer. This is the "absorption phase." After the alcohol enters the bloodstream, the body then begins to filter it out. This is the "elimination phase."

 
In order to even conceptually do a retrograde analysis, the person must be in the elimination phase. Otherwise, as you can see in the above graph, if a person was at hour 3, their BAC was actually lower in hour 1. However, the retrograde would instead make it higher. This is true anywhere in the absorption phase.

The second issue is Widmark's is based on averages. While averages work in some settings, when determining someone about an individual, averages are generally not a good metric. People process alcohol differently and eliminate it at different rates. The equation knows nothing about you.

The use of Widmark's formula and retrograde analysis has drawn criticism from scholars in the area of breath and blood alcohol analysis, yet it still used in courtrooms across the nation to attempt to convict people who are under the legal limit.