Retrograde Extrapolation in DUI cases
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The technique of retrograde extrapolation can be a very valuable tool in Massachusetts Operating Under the Influence cases. Retrograde extrapolation is a process used to determining blood alcohol content at the time of operation by working backwards from the time the breath test is taken. It is based upon the use of the average absorption and elimination rates for alcohol. Normally, this process is done by a properly qualified expert witness who has sufficient education, training, background, and experience to give an opinion as to the defendant’s blood alcohol concentration at the time he or she was alleged to have been operating a motor vehicle based on a breath or blood test conducted some time after the DUI arrest. Retrograde extrapolation satisfies the “general acceptance” standard for the admissibility of scientific evidence.
However, in Massachusetts, expert testimony regarding retrograde extrapolation is not required in order to admit breathalyzer results against a defendant charged with operating a motor vehicle with a blood alcohol level at or above .08, where the breathalyzer test has been administered within a reasonable time of the operation of a motor vehicle. In one particular case, the breathalyzer was administered more than an hour after the defendant operated the vehicle, and the Massachusetts Supreme Judicial Court stated that it was reasonable to rely on a breath test administered up to three hours after operation. See Commonwealth v. Colturi, 448 Mass. 809 (2007). “Extrapolation to that earlier time [of the offense] by a scientifically acceptable or reliable method … might logically be thought to be of assistance to a jury.”
Massachusetts courts have ruled that retrograde extrapolation analysis is a sufficiently reliable method of determining blood alcohol levels and admissible to show defendant's blood alcohol content at a time prior to the actual breath or blood test. Prosecutors have used this procedure to establish that a driver’s BAC was higher at the time of operation than when he took the breathalyzer test some time later and it has also been successfully used by DUI defense lawyers to show that the driver’s blood alcohol content was lower at the time of operation than it was when the breath test was administered. Thus, when properly conducted, retrograde extrapolation can be a powerful tool in DUI cases, especially when the driver has been charged with a violation of the per se portion of the DUI statute, which makes it a crime to operate a motor vehicle on a public way or where the public has a right of access with a blood alcohol content of .08 or more.
In summary, retrograde extrapolation can sometimes be used to get a not guilty verdict in DUI cases both with and without an expert witness, especially in cases where the BAC results are borderline.