How does the prosecution prove intoxication at the time of driving in a Driving Under the Influence of Intoxicants (DUII) case? Most people know that it is illegal to drive a car if their blood alcohol content (BAC) is at or above 0.08%. The prosecutor can also prove intoxication, without a BAC above 0.08%, if she can show that the person was impaired to a noticeable and perceptible degree. Either way, however, the prosecutor must show that the intoxication was present when the person was driving, even though most people don’t take a breathalyzer test to measure BAC until an hour or more after they were driving. Many prosecutors rely on retrograde extrapolation.
Intoxication Proven through Retrograde Extrapolation
Many prosecutors rely on experts to testify regarding “retrograde extrapolation” to prove intoxication at the time of driving. This is a scientific term for calculating how quickly alcohol leaves a person’s body (called “dissipation rate”) compared to the amount of time since driving in order to tell where the person’s BAC was when the person was driving. In other words, even if a person blows a 0.07% BAC an hour later at the police station, a prosecutor can have a scientist testify that the person was well above a 0.08% BAC at the time of driving and so can prove intoxication that way.
Retrograde Extrapolation is a Dubious Practice
Use of retrograde extrapolation to prove what a person’s BAC was at an earlier time is common in many DUII trials. Despite it’s popularity, however, leading experts in the area have referred to it as a “dubious practice.” There are many different factors that determine how quickly alcohol leaves a person’s body, such as body size, metabolism, and whether the person recently ate a meal. These factors make figuring out a person’s BAC in the past a guessing game much of the time.
Challenges to Retrograde Extrapolation
A recent case in the Court of Appeals challenged the use of retrograde extrapolation testimony in trial. The defendant in that case argued that the practice is too unreliable and so shouldn’t be allowed in as scientific evidence. The argument is that a jury will rely on it, because it sounds scientific, even though it’s no more than a guessing game. Unfortunately the Court of Appeals said that the issue wasn’t preserved, so didn’t rule on whether retrograde extrapolation is admissible as scientific evidence. The case does raise good issues, however, and it paves the road for another defendant to properly challenge the practice. We can expect the Court of Appeals to rule on whether retrograde extrapolation can be used in the near future – could it be your case? If you have a DUII case pending against you where you blew below a 0.08%, contact us to discuss the specifics of your case.