Retrograde Extrapolation in the Context of Arizona DUIs

retrograde extrapolationIf you are charged with DUI in Arizona, the state’s unique retrograde extrapolation law will come into play.  The science and math of this law have the potential to make or break your case.  However, most motorists have never heard of retrograde extrapolation.  Let’s take a look at retrograde extrapolation and its importance to those charged with DUI.

Arizona’s Retrograde Extrapolation

In layman’s terms, retrograde extrapolation is a math formula that supposedly has the potential to accurately measure blood alcohol content, commonly known as BAC.  This formula allegedly determines the driver’s BAC at the time of the accident or within two hours of driving when blood is taken by way of a warrant issued by a judge.  The word “retrograde” refers to the time.  The word “extrapolation” refers to the blood sample’s BAC result.

The Issue of Time

Prosecutors are tasked with essentially traveling back in time to show the accused party was inebriated at the time of the accident.  The state is required to obtain a blood sample from the allegedly drunk driver within two hours.  Otherwise, the BAC level has the potential to be inaccurate.  However, there is the potential for delays to occur in testing, especially in the early morning hours.  If such delays occur, the state is allowed to prosecute the supposedly drunk driver under retrograde extrapolation.  Critics of retrograde extrapolation argue this approach requires considerable guesswork.

The Math Component of Retrograde Extrapolation

Here’s how retrograde extrapolation works: the prosecution adds the BAC result at the time at which the test was performed to the hourly rate at which alcohol is eliminated from the blood.  The math part of retrograde extrapolation considers the number of ounces of alcohol consumed, the individual’s gender constant (.66 for women and .73 for men), the amount of time passed since the drinking commenced and the individual’s weight.  This formula provides a supposedly accurate estimation of the driver’s BAC level at the time of the traffic stop or for our state’s purposes.  This estimation gauges the BAC level within two hours of the time at which the driver operated the motor vehicle.

Science’s Role in Retrograde Extrapolation

Retrograde extrapolation also has a science aspect.  Once the individual begins drinking, it will take half an hour or possibly longer before the blood absorbs the alcohol.  Alcohol concentration will increase all he way up until hitting maximum concentration, referred to as the peak, when drinking stops.  Once the peak is determined, alcohol concentration starts to decrease as the body absorbs alcohol fairly slowly yet eliminates it hastily, generating an elevated BAC.

There are a number of factors that determine the rate at which alcohol is absorbed and eliminated.  Everything from the driver’s metabolism to his or her height, the type of drink consumed and the amount consumed matter a great deal.  Furthermore, some people are more tolerant of alcohol than others.  Those who have been drinking heavily for years have a high tolerance level compared to those who merely drink in social settings.

The Problem With Retrograde Extrapolation

Arizona police officers will not ask drivers what they have had to eat, how long ago they consumed it, their metabolic rate, etc.  However, relying on the average times and figures for such factors is problematic in the context of retrograde extrapolation.  Each individual processes alcohol at a unique rate.  Retrograde extrapolation is generally considered somewhat unreliable yet it a component of state law.  If you are charged with DUI, you need a talented DUI defense attorney on your side to poke holes in the prosecution’s retrograde extrapolation and ultimately clear your name.

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