Criminal Defense Lawyer | DWI, Drug, Theft & Assault Charges

By Collin County Criminal Defense Lawyer Jeremy Rosenthal

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You can be acquitted of DWI even if your breath and/or blood score is above a 0.08 and this is why:

Texas Penal Code 49.04 defines Driving While Intoxicated in the following manner, “A person commits an offense if the person is intoxicated while operating a motor vehicle in a public place.” (Emphasis mine)

Texas Penal Code 49.02 (A) and (B) legally define intoxicated as, “not having the normal use of mental or physical faculties by reason of the introduction of alcohol, a controlled substance, a drug, a dangerous drug, a combination of two or more of those substances, or any other substance into the body; or  having an alcohol concentration of 0.08 or more.

As you can see, your blood alcohol concentration must be at 0.08 or more while you are operating a motor vehicle… not an hour or two hours after you operated a motor vehicle for the State to convict  you based on that definition.

This brings us to the concept of “retrograde extrapolation.”  While that sounds like a NASA term, retrograde extrapolation is the science behind trying to determine what someone’s blood alcohol concentration was several hours in the past.

Several things factor into retrograde extrapolation.  A person with the proper scientific background (usually the State’s breath test expert) can calculate what someone’s approximate blood alcohol concentration was at the time of driving based on factors such as height, weight, gender, type of alcoholic beverage consumed, and type or quantity of meal and time of the last drink.

It is not uncommon for the witness (typically the State’s breath test expert) to estimate that a person’s blood alcohol concentration was below 0.08 at the time of driving (or that the person’s BAC was actually higher while driving than it was at the time of testing) based on the retrograde extrapolation facts.  Jurors, then, may have a reasonable doubt as to whether the driver had a BAC of 0.08 or more while driving even though their breath test scores are above 0.08.

As a point of caution — retrograde extrapolation is based on scientific principals and Courts have limitations on what jurors will be allowed to considered as scientific testimony.  Only cases where the BAC score is reasonably close to 0.08 may cause the BAC to extrapolate low enough to make a difference.  For instance, it doesn’t help your case if the State’s expert witness says “instead of a 0.14, the subject was possibly at 0.13 at the time of driving.”

Police routinely question people (usually after the breath test) about what they had eaten, when they last ate, when they last drank, etc.  These questions are for the purposes of later retrograde extrapolation.  These questions are testimonial in nature and you have the right to refuse to answer them — which is typically the safer course.

Finally, a jury can simply have a reasonable doubt as to the validity of the breath test score — regardless of what it is.  If the jury has a reasonable doubt that the person on trial is intoxicated (on all the legal definitions of 49.02) — the verdict would be not guilty just the same.

*Jeremy Rosenthal is Board Certified in Criminal Law by the Texas Board of Legal Specialization and licensed by the Supreme Court of Texas. Nothing in this article is intended to be legal advice.  For specific legal advice you should consult an attorney.