In Arizona, officers must draw your blood within two hours of you being pulled over for suspicion of DUI if they wish to use the blood results as evidence against you. That’s because the law says you cannot have a prohibited alcohol concentration within two hours of driving or being in actual physical control of a car. The police are aware of this short window and will try to draw your blood as quickly as they can. However, in some cases, a driver’s blood is not taken until 2 hours after the stop. In fact, it is not unusual for testing to occur outside the 2-hour window.
Even if the police miss the two-hour window on drawing your blood, they can still try to use the results as evidence against you by relying on something called retrograde extrapolation. This scientific process determines if you had a BAC of .08 or higher within two hours of driving. Usually, the State will have a chemist testify using the blood result and guess backwards as to what your BAC would have been within two hours of driving.
However, retrograde extrapolation has been criticized by many experts in the field. Some argue that this method requires a chemist to make several assumptions. For example, chemists often assume that the alcohol was completely absorbed at the time of testing, or that the alcohol curve was charted correctly, or that the alcohol in your system was eliminated at an average rate. These are major assumptions that may be wrong and affect the results of the chemist’s BAC estimation.
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