DUI Myth Book - The Guilt Myth - BAC Level

At trial, one of the most important pieces of evidence against you is the chemical test that you took at the police station. But 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 under the influence at the police station. And it is safe to say that there are no laws on the books that make it illegal to be under the influence 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 means using the rate at which an average person metabolizes alcohol and the breath test result to estimate the person's blood alcohol at the time of driving. For most people, their BAC falls over time as the body breaks down the alcohol.

But what if your metabolism is different from the "average person"? Studies have shown that it is relatively common for a person's metabolism to vary substantially from the norm. We don't need 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.

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.

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 first problem with these laws is that they are based on an untruth. BAC is hardly ever the same at the police station as it was in the car. The second problem, and potentially more important that the first, 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 chemical test results in DUI 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.

These kinds of laws have been ultimately responsible for countless DUI convictions. Faith in the unreliable and error-prone methods of breath testing and retrograde extrapolation has resulted in the convictions of countless people who should never have been arrested in the first place. It is imperative that you find an attorney who knows how to prevent you from becoming a victim of bad luck, which so often can mean the difference between a correct and incorrect test result.

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