Expert’s assumption found faulty in BAC calculation

DUI prosecutions often depend on a collection of items of evidence, which can include observations by law enforcement personnel and other eyewitnesses, as well as complex technical information. In the case of People v. Floyd, the Illinois Appeals Court found that the state's expert witness gave highly prejudicial evidence of the defendant's supposed intoxication level at an earlier time calculated from a much later test.

OnStar call in parking lot and other hijinks

The defendant's evening began at a family aquatic center parking lot, when she made a 911 call at 7:30 pm from the OnStar system in her vehicle. While telling the operator that she had a boyfriend who was violent and also wanted to "hit and rob her" and that a man in her vehicle had made her perform sexual acts, she was also heard telling the man in the car that she had been "drinking since I've been here."

The police officer dispatched to the parking lot observed the defendant and a man arguing outside a car, and told the defendant not to drive because he thought she was drunk. After the officer left, the manager of the center saw that the defendant was back in her car, listening to loud music. An hour later, a building manager saw a man pounding on the defendant's car, the car started into a low-speed chase in the lot, and then went back to its original parking spot as the man retreated.

Late BAC test, then calculation of past intoxication

The defendant refused to take a breath test once police arrived, tried to walk away, and was arrested at the scene. She failed a field sobriety test at the police station, and when her BAC level was finally taken at 10:30 pm, it registered .069, below the level of .08 at which a person is presumed to be legally drunk. Nonetheless she was convicted of aggravated DUI and resisting arrest.

At trial, the state introduced a forensic toxicologist to explain the theory of "retrograde extrapolation calculation," and how he determined that the defendant's BAC at 9:10 pm was between .085 and .095. In making this calculation, the expert relied on assumptions of the defendant's absorption rate and whether she was in the "elimination phase" during which alcohol is no longer being absorbed into the body.

Expert's calculation was prejudicial

The state's expert stated the information needed to make a retrograde extrapolation calculation of a person's BAC in hours prior to any tests. Some of those factors include the defendant's height and weight, what kind of alcohol was consumed, how long the person was drinking and the type and amount of food in the person's stomach. The expert did not have any of that information, and just assumed facts about the defendant in stating that she was in the elimination phase when tested.

In determining that the expert's testimony was inherently unreliable, the appeals court said it "invited the jury to convict defendant on the basis of a supposedly high BAC." Finding the testimony about the retrograde extrapolation calculation to be prejudicial to the defendant, and that the trial court should have excluded it, the court granted her a new trial.

Testing for intoxication can go wrong in numerous ways-the test may be administered incorrectly, the machinery may not be accurate, or, as in this case, an expert may testify to an intoxication theory without having the proper information. If accused of drunk driving, you should have representation by attorneys familiar with breath and blood testing and with local courts and police.