In order to obtain a conviction for a DUI offense, prosecutors must prove beyond a reasonable doubt that the defendant had a blood-alcohol level at or above the legal limit at the time the defendant was operating a motor vehicle. Because police usually rely on a non-portable breath alcohol analyzer or a blood test to prove intoxication, defendants are often not tested until several hours after they have been stopped by police. When a test result after arrest demonstrates a blood alcohol level below the legal limit at the time of the test, police and prosecutors rely on a scientific technique known as retrograde extrapolation to estimate a defendant’s blood alcohol content at the time they were operating a motor vehicle. The Arizona Court of Appeals recently affirmed the aggravated DUI conviction of a defendant whose blood alcohol level had been estimated using retrograde extrapolation.
Retrograde extrapolation is a technique used by police departments and prosecutors to determine and prove a person’s blood-alcohol level at the time they were operating a vehicle, as opposed to when the sample was actually collected. Crime lab chemists will apply a formula that approximate the average rate of decline and a person’s blood-alcohol level and use that to estimate a person’s blood-alcohol level at a time prior to the sample being taken. Although Arizona courts accept retrograde extrapolation as a method of proving intoxication, it is far from a perfect science.
The results of a retrograde extrapolation can be inaccurate for several reasons. First, each person metabolizes alcohol at a different rate, and simply applying the average rate of metabolization to every sample guarantees some inaccurate results. Furthermore, if a defendant consumed alcohol shortly before their arrest, their blood alcohol level may increase rather than decrease in the time before their sample is taken. If standard retrograde extrapolation is applied, the result would be inflated and inaccurate.