Experts contest the validity of retrograde extrapolation in the DUI trial of Cody Austin Shirah.
In September 2016, a group of Ohio softball players were in Bay County, Florida for the World Sports League World Softball Championships. As they were on their way back to their hotel from their tournament, Cody Austin Shirah ran a stop sign and crashed into their minivan. Two of the men died at the scene; two other died as a result of their injuries from the crash.
Shirah was charged with four counts of negligent manslaughter involving a motor vehicle, or DUI manslaughter and driving without a license.
The Blood Tests
Carol Seagle, a toxicology analyst with the Florida Department of Law Enforcement, testified that a sample of Shirah’s blood was taken about 3 hours and 45 minutes after the crash. Shirah’s blood had a blood alcohol content of 0.078 percent. The legal limit is 0.08. Seagle testified that Shirah’s blood alcohol content was likely between 0.09 percent and 0.14 percent at the time of the crash.
Barry Funk, a forensic toxicologist for the defense contested the methods that Seagle used to reach her conclusion. Funk explained that the process of retrograde extrapolation involves too many factors to be trusted, including medical history, injuries, and medicines that have been taken. Funk said, “All things have to be taken into consideration with retrograde extrapolation…You can’t count them out as factors. It makes a difference to conclude what alcohol content would’ve been at the time of the incident.”
Despite Funk’s attempt to question the validity of the use of retrograde extrapolation, the prosecutor was able to elicit Funk’s admission that the blood drawn at the hospital one hour after the crash amounted to 0.125 percent alcohol content. Additionally, Shirah’s girlfriend, who had been a passenger in his truck had a blood alcohol content of 0.145 percent. There were also beer cans found around the area where Shirah’s truck had overturned.
Retrograde Extrapolation Explained
The use of retrograde extrapolation to determine blood alcohol content is based on the assumption that people eliminate alcohol at a fixed rate of between 0.01 grams and 0.02 grams per deciliter of blood per hour. For a retrograde extrapolation calculation to be accurate, a person must metabolize alcohol at the normal rate and the person must be in the postabsorption phase, which typically occurs 15 to 90 minutes after a person’s last drink.
However, each person’s absorption rate varies. Many factors, including the type of food eaten, type of alcohol consumed, and the length of time during which the drinking occurred will affect the rate of absorption. Because retrograde extrapolation is uncertain, blood alcohol test results are only presumed to be valid if the blood sample is drawn within three hours after the test subject was driving. The longer the delay in obtaining a test sample, the more speculative those conclusions are likely to be.