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What Is Retrograde Extrapolation for DUI Testing? Is It Junk Science?

Draft Beer 20150701 202142 1The general consensus is that alcohol is metabolized by your body at a rate of about .015 per hour. So for someone who was not safe to drive, at a .08 BAC, it would take a little over five hours to metabolize all of the alcohol in his or her body. This rate is used for something called retrograde extrapolation, a fancy term that essentially just means guessing what your BAC was over the course of the last few hours.

Police use retrograde extrapolation most often when a person has not had their blood tested yet, but they are suspected of a DUI. If a driver refuses testing when pulled over, then by the time he or she reaches the police station, his or her body has metabolized some of the alcohol. Say a man is taken to the police station and tested two hours after his DUI stop. He has a .06 BAC at the time of testing. Cops use the metabolizing rate of alcohol to determine that, at the time of his DUI stop, his BAC was .09 – above the legal limit. That’s retrograde extrapolation.

Don’t you find it a little odd that you could be slapped with a DUI for what amounts to simply an educated guess of your BAC? Whatever happened to proving your impairment “beyond a reasonable doubt?” To even have a possibility of extrapolating a person’s past impairment, you’d need to know a ton of things. You’d need to know:

  • That the results you are extrapolating off of are accurate – they aren’t always, as a lot of substances will throw off a breathalyzer
  • The person’s gender, weight, lean body mass, age, true height, mental state – all things that can affect how quickly alcohol metabolizes
  • The type of alcohol consumed
  • Whether the person had food in his or her stomach, and how much
  • When the person started drinking, when he or she stopped drinking, how quickly he or she was drinking, whether the drink was mixed
  • Whether the person may have a gastrointestinal condition that could affect how food and drink are absorbed

Those are just a few examples. The truth of the matter is that retrograde extrapolation is, in the end, nothing but an educated guess. However, it is widely used, and because of that, you need to be prepared for the potential penalties. If you are facing a Colorado DUI, speak to a drunk driving defense attorney.

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