Retrograde Extrapolation Evidence
The primary indicator of whether a person has had too much to drink is their blood alcohol content (BAC) level. The BAC describes the concentration of alcohol in a person’s blood expressed as weight per unit of volume. For example, at 0.10 percent BAC, there is a concentration of 100mg of alcohol per 100 ml of blood.
Retrograde extrapolation is the process of estimating an alcohol content at an earlier time from a measured alcohol content at a later time. In most traffic cases, this means providing an estimate of BAC at the time of the offense based on the measured BAC at the time a sample of blood, breath or urine was obtained. The most important parts of a person’s drinking history for purposes of retrograde extrapolation are alcohol consumption in the hour prior to the offense and any consumption after the offense but before the test. Knowing where the defendant last consumed alcohol and its distance to the place of arrest can provide a minimum estimate of the time of the defendant’s last drink. Similarly, developing a time line and assessing the availability of alcohol from the time of a crash to the time that police arrested the defendant can help in determining the likelihood of post-crash alcohol consumption.
The concept of retrograde extrapolation began in 1932 with the work of Swedish scientist Dr. Erik Widmark. Dr. Widmark conducted many studies on the rate of elimination of alcohol by the body. He was the creator of the Widmark factor, a mathematical factor used to correlate blood alcohol and the amount of alcohol consumed. Dr. Widmark was the first scientist to develop a reliable test sufficiently sensitive to allow for the measurement of the amount of alcohol in blood using a sample as small as a drop.
In most DWI cases, there is a difference between alcohol content at the time of driving and the time of the chemical test. Many prosecutors rely on retrograde extrapolation to argue that the DWI offender’s blood alcohol content was higher at the time of the arrest or the time of driving. In terms of DWI defense, numerous courts have allowed defendants to present retrograde extrapolation evidence. This evidence may be presented to rebut the prosecution’s blood alcohol test results. In other cases, the defense has argued that the BAC is inadmissible at trial to prove a statutory per se DWI violation unless the prosecution introduces evidence to relate the BAC back to the time of the arrest.
Retrograde extrapolation evidence can be used in DWI cases by the defense to create reasonable doubt as to the defendant’s guilt. However, when the prosecution attempts to offer retrograde extrapolation evidence as proof of the defendant’s guilt beyond a reasonable doubt, many attorneys believe that the state’s expert should be thoroughly cross-examined on the assumptions that underlie their opinion that the defendant’s BAC was higher at the time of driving.
In New York, the courts have admitted retrograde extrapolation evidence to reverse some DUI convictions. In the case of People v. Mertz, the court reversed a DWI conviction and held that a defendant could introduce retrograde extrapolation evidence to show that his BAC was below the legal limit while driving. The court held that scientifically accepted facts regarding extrapolation might prove that the defendant’s BAC may have been rising for two hours after driving and therefore might have been lower while driving than during testing. The court held that BAC evidence introduced by the prosecution in DWI cases may be rebutted by the defense with retrograde extrapolation testimony, and the courts may not foreclose a defendant’s opportunity to do so.
This guest post is brought to you by the attorneys at the law firm of Anelli Xavier, a Buffalo NY DWI attorney.