What Is Retrograde Extrapolation in an Arizona DUI Case?

When someone consumes alcohol, their blood-alcohol content (BAC) will increase over time, before it starts to decrease as the alcohol dissipates from their blood. For many Arizona DUI offenses, the prosecution must prove that the defendant’s blood was above the legal limit. Thus, police officers will often try to take a driver’s blood as quickly as they can; however, in some cases, a driver’s blood is not taken until a later time. Typically, blood must be drawn within two hours of the time when the defendant was driving.

Recently, a state appellate court issued a written opinion in an Arizona DUI case discussing a process called “retrograde extrapolation” by which a chemist can estimate what a person’s BAC was at a specific time by looking at what their BAC was at a later time. The process is used by prosecutors to estimate what a defendant’s BAC would be at the time they were driving. Prosecutors will especially rely on this technique when they were unable to take a defendant’s blood within the two-hour time frame

According to the court’s opinion, witnesses observed the defendant get into a car accident between 4 and 6 p.m. After the accident, the witnesses noticed that the defendant smelled of alcohol and seemed off balance. Police officers arrived on the scene at 8 p.m, and the defendant’s blood was taken at 9 p.m. The results indicated that the defendant’s BAC was .336. Because the defendant’s blood was not taken until between three to five hours after the accident, prosecutors called an expert witness to explain the concept of retrograde extrapolation, and provide the jury an estimate of the defendant’s BAC at the time of the accident.

The defendant was ultimately found guilty, and appealed the admissibility of the retrograde extrapolation evidence. Specifically, the defendant claimed that the evidence was not relevant because the specific statute he was charged with violating did not require proof of a certain BAC, and only required the prosecution to show that he was generally impaired.

The court rejected the defendant’s argument, finding that the retrograde extrapolation evidence was relevant to the question as to whether the defendant was under the influence at the time of the accident. The court noted that law provides that, once the prosecution provides evidence that a motorist had a BAC of more than .08, there is a presumption that the motorist was intoxicated. Here, the court held that the evidence was relevant because it helped establish that the defendant’s BAC was over .08 at the time he was driving.

Have You Been Arrested for an Arizona DUI Offense?

If you have recently been arrested and charged with an Arizona DUI offense, contact Attorney James E. Novak for immediate assistance. Attorney Novak is a veteran Tempe DUI defense attorney with extensive experience defending clients against all types of DUI charges. He is both a skilled litigator and negotiator, which helps him consistently obtain favorable results for his clients. To learn more, and to schedule a free consultation to discuss your case with Attorney Novak, call 480-413-1499 today.