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The State Can Prove You Were Over Limit at the Time of Driving Law


In most DWI breath test law cases, the State will attempt to convince the jury to find a person guilty based on a failed breath test.  In every one of these cases, the State will use a breath test that was given some time after the arrest was made.

It is important to remember that officers do not have scientifically valid breath test machines in their vehicles when they make an arrest for DWI.  Instead, the officer must transport the citizen accused of DWI to what is known as the “in toxilyzer room” which is usually located in a city or county jail.  The length of time between an arrest on the side of the road and a breath test score that is given in an in toxilyzer room is an important factor in every DWI breath test prosecution.

The law requires that the state must prove a person was intoxicated at the time of driving, not 30 minutes, one hour or in many cases even longer when he or she gives a breath test at a jail facility.  In order to relate a failed breath test back to the time of driving, a State’s expert must use what is known as Retrograde Extrapolation.

Retrograde Extrapolation is the scientific process used to take the score of a breath test and then back it up in time to show what a person’s BAC was when they were driving.  However, in order to properly calculate using retrograde extrapolation the State’s expert needs to know the following information:

  1. The person’s gender;
  2. Their weight;
  3. The time they had their first drink;
  4. The time they had their last drink;
  5. The time of the stop (arrest);
  6. The type of alcoholic beverage consumed;
  7. The type of  food a person ate and when they last ate;
  8. The time the breath test was given;
  9. The result of the breath test.

Often time, the state does not have all of the information for their expert to properly retrograde a breath test result to the time of driving.  In these cases, a successful argument can be made by the defense attorney that the State did not satisfy its burden of proving that the citizen accused was intoxicated at the time they were operating a motor vehicle, and therefore, they should be found not guilty of driving while intoxicated.

It is important to do a thorough review of the police report and video of a DWI investigation to determine if the State will have problems with retrograde extrapolation in a DWI breath test case.  This is one of the most successful defensive arguments and can, in some cases, lead to a not guilty verdict.

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