If You Can't Prove It, Make the Defendant Disprove It
Information courtesy of Lawrence Taylor - DUIblog
The drunk driving laws make it a criminal offense to drive a vehicle while under the influence of alcohol (DUI) or while having a blood-alcohol concentration (BAC) of .08% or higher. It is not, however, a criminal offense to be under the influence or to have a BAC of .08% while taking a breath test in a police station an hour or two AFTER driving. So how does the prosecution prove what the BAC was when the defendant was driving?
Its a problem. You can try to guess what the BAC was in a DUI case by projecting backwards, using average alcohol absorption and elimination rates, but its only a very inaccurate guess. The process is called "retrograde extrapolation" -- a fancy name for trying to guess backwards. The problem is that everyone has a different metabolism, and even a given person will metabolize alcohol at different rates depending on many variables. In one study, for example, researchers found a wide range of matabolism rates: some individuals can absorb alcohol and reach peak blood-alcohol levels ten times faster than others. Dubowski, "Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects", Journal on Studies of Alcohol (July 1985). As a result, scientists have concluded that the practice of estimating earlier BAC levels in DUI cases is highly inaccurate and should be discouraged. From the recognized expert in the field, Professor Kurt Dubowski of the University of Oklahoma:
"It is unusual for enough reliable information to be available in a given case to permit a meaningful and fair value to be obtained by retrograde extrapolation. If attempted, it must be based on assumptions of uncertain validity, or the answer must be given in terms of a range of possible values so wide that it is rarely of any use. If retrograde extrapolation of a blood concentration is based on a breath analysis the difficulty is compounded." 21(1) Journal of Forensic Sciences 9 (Jan. 1976).
So, Mr. Prosecutor, youve got a Breathalyzer reading of .10% an hour or two after the driving and the scientists say you cant accurately project that BAC back to the