Retrograde Extrapolation and DUI in Illinois

The timing between a stop and subsequent arrest for DUI and breath or blood analysis to determine the presence and amount of alcohol can play a crucial role into whether or not prosecutors can meet their burden and obtain a conviction for DUI. Often times, due to numerous circumstances that may arise, a breath analysis at the police station may come hours after an individual was driving while allegedly under the influence. Yet, this delay in testing will not necessarily bode well for the driver. For instance, if someone was stopped and arrested for DUI and two hours later a breath analysis revealed a blood alcohol concentration (BAC) of .16, it’s fair to presume that at the time the individual was driving, he was over the .08 limit. This argument assumes that the individual had not consumed any other alcohol in the two-hour time span, among many other assumptions.

However, some situations are not as clear-cut as described in the example above. Let us assume the same fact pattern above, but this time the driver’s blood alcohol concentration is at .06 at the time of testing. What is the result now? Will the DUI charge of testing .08 or above be dismissed? 625 ILCS 5/11-501(a)(1) states that no person shall drive or be in actual physical control of a motor vehicle when that person’s blood alcohol concentration is .08 or more. Prosecutors can still file charges and secure a conviction on this charge even if a person’s blood alcohol concentration at the time of testing is under the legal limit. What the prosecutors would use to prove their case is called “retrograde extrapolation.”

Retrograde extrapolation is based on the theory that a person’s blood alcohol concentration, obtained from analysis of an individual’s blood or breath at a particular time, can be extrapolated back to an allegedly higher blood alcohol concentration at the time of the person’s operation of a vehicle. In order to use retrograde extrapolation, the prosecutors would need a forensic toxicologist to testify. The forensic toxicologist would calculate what the driver’s blood alcohol concentration was at the time the driver was operating the vehicle.

Nonetheless, there would be many evidentiary rules the prosecutors and forensic toxicologist would need to meet in order for the toxicologist to testify at trial what he believed the driver’s BAC was at a certain point. In order for the toxicologist to testify, the prosecution must show that information used in the toxicologist’s calculation was reliable and that he or she performed the calculations in a method that is generally accepted within the field of forensic toxicology.

If the toxicologist takes into account a driver’s age, weight, height, amount of alcohol consumed, type of alcohol consumed, alcohol absorption rate, type and amount of food consumed before, during and after drinking, a court may find the toxicologist’s calculations more reliable and thus would let the toxicologist opine on the person’s BAC. If however, these key variables are not used in the toxicologist’s calculations, a court may find the toxicologist’s opinions less reliable, and would not allow the calculation be used as evidence against the driver, which was the case in the recent appellate court decision in People v. Floyd.

As you can see, when it comes to retrograde extrapolation, the formula and information used by an expert witness prosecutors hire to testify against a driver can be complex and involve intricate rules of evidence. If found in a situation where one is faced with a DUI charge, it is advisable for that person to hire an attorney experienced in DUI law, rules of evidence and expert witnesses. An experienced attorney may even be able to argue a “reverse retrograde extrapolation” theory, that is, if a person was stopped and within a short time afterwards tested at a .08 BAC, or slightly above, and it can be shown that the driver had consumed alcohol in a very brief time before driving, that at the time the driver was stopped, his or her BAC was under the legal limit. To date, no case law has addressed the issue of a reverse retrograde extrapolation and it is unknown how that argument would stand in a court of law.

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