How do prosecutors prove that a person was intoxicated while driving? The answer is: they don’t, because they don’t have to.
Consider for a moment that when a suspect takes a chemical test at the police station, the only thing the test actually proves—leaving aside the myriad ways in which these tests can go wrong—is that the suspect is drunk at the police station. And it’s safe to say that there are no laws on the books that make it illegal to be drunk at a police station.
So how can you tell that the person was above the legal limit an hour or two before taking the test. The answer is a process called “retrograde extrapolation,” which involves estimating what a suspect’s blood alcohol level (BAC) was based on what it is two hours later, given how fast the average person metabolizes alcohol. For most people, their BAC falls over time as the body breaks down the alcohol.
But what if your metabolism is different from that of the “average person”? Studies have shown that it is relatively common for a person’s metabolism to vary substantially from the norm. And it’s not as though we needed a study to tell us this. Every one of us has a friend who can eat twice what we can and still stay thin, or a friend who eats half what we do but is somehow our same size.
Professor Kurt Dubowski of the University of Oklahoma has this to say about differing metabolism and retrograde extrapolation:
“It is unusual for enough reliable information to be available in a given case to permit a meaningful and fair value to be obtained by retrograde extrapolation. If attempted, it must be based on assumptions of uncertain validity, or the answer must be given in terms of a range of possible values so wide that it is rarely of any use. If retrograde extrapolation of a blood concentration is based on a breath analysis the difficulty is compounded.” 21(1) Journal of Forensic Sciences 9 (Jan. 1976)
In other words, if you don’t take into account a person’s metabolism, “retrograde extrapolation” is a shot in the dark.
Not only does “retrograde extrapolation” assume that everyone’s metabolism is the same, it also assumes that a suspect’s BAC is always lower at the police station than it was in the car. While probably true in the majority of cases, it is not always something you can count on.
In order to be even remotely accurate, chemical tests at the police station must be taken after the suspect has reached what’s called the “post-absorptive” state—a state of equilibrium that is reached when the person is no longer absorbing alcohol into the blood stream. During the “absorptive state,” on the other hand, BAC is often rising. This means that if you test a suspect too soon, the BAC result might actually be higher at the police station than it was in the car.
And what determines when someone has reached the post-absorptive state? Average metabolism! The moral of the story is that if you are tested too early for your metabolism, not only will “retrograde extrapolation” produce an inaccurate result, but it will estimate in the wrong direction!
In response to these difficult issues of proof, many states have tried to simplify matters by passing laws governing how the chemical tests should be interpreted. Many states have laws that say that, unless you can prove otherwise, the BAC result at the police station will be assumed to be the same as it was when driving. These laws are an attempt to fashion a compromise between two facts: 1) retrograde extrapolation is inaccurate (as the defense would point out) and 2) BAC is almost always lower at the police station (as the prosecution would respond). So let’s just split the difference.
The problem is that these laws are based on an untruth. BAC is hardly ever the same at the police station as it was in the car. Another important thing to note about these laws is that they shift the burden of proof. In our legal system, the prosecution has the burden of proof. In other words, a suspect is presumed innocent unless it can be proven otherwise beyond a reasonable doubt. But when in comes to the question of how to interpret the results chemical test in DUI/DWI cases, the burden of proof falls on the defense. If you were arrested for drunk driving and your test result registered above the legal limit, it is up to you to prove that the results are inaccurate, as they so often are.