In a recent case from the Nevada Supreme Court, State v. Eighth Judicial District Court, 2011 WL 6840685, the court decided that a blood sample taken from a driver over two hours after he was involved in a collision was unreliable to the point that to allow a jury to hear it would unfairly prejudical. The reasoning of the court was based on the use of "retrograde extrapolation," a methodology by which an expert attempts to determine the approximate level of alcohol in a person's blood at the time of stop for suspected drunk driving. This methodology is based on the the blood or breathe sample taken later, and the consideration of a number of factors that can lead to what is no more than an educated guess about the blood/alcohol level at the time of the stop. Those factors include: the amount of time between a person's last drink and the blood test, the amount and type of alcohol consumed, the time period over which alcohol was consumed, and personal characteristics such as age, weight, alcohol tolernance, and food. Of these, the court said that the amount of food consumed, according to one of the experts who testified, was one of the most important factors. In this case, it was unknown. Also unknown were the defendant's age or height, his regular drinking pattern, or his emotional state after the collision. The court also emphasized that only one sample was taken, and it was taken over two hours after the collision. This is important, because having more than one sample can help determine if the person was absorbing or eliminating alcohol at the time the blood sample was taken. The court also quoted at length from a Texas case on the same subject, State v. Mata, 46 S.W.3d 902 (Tex.Crim.App. 2001), which includes even more factors that need to be taken into consideration in order to determine the reliability of retrograde extrapolation.
So, even if a person has been arrested for DWI and a blood sample has been taken, the prosecutor has some hurdles to overcome to prove that the blood test is reliable, especially if a significant amount of time has expired between the time of the original stop for the suspected drunk driving. (Not the time of the arrest, for this could be somewhere 15 minutes or an hour after the stop.) Just because the prosecutor may present testimony from an expert on the estimated level of alcohol in the blood at the time of the stop, that testimony may be unreliable due to a number of unknow factors about the person, the amount of alcohol consumed, what the person had to eat and when, his or her weight, the time the last drink was consumed, etc. Just having a high breath test or blood test is not always the end of the ballgame in a DWI case.
