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“Retrograde extrapolation” in DUI prosecution, P.1

For police officers, time is of the essence when it comes to getting an accurate reading of a suspected drunken driver’s blood alcohol content. Because alcohol dissipates in the blood, officers ordinarily work to get a suspect tested as soon as possible after arrest. In some cases, though, a test may not be administered until hours after the suspect operated a motor vehicle.

When this occurs, prosecutors seeking to gain a conviction may turn to a technique called retrograde extrapolation. The technique has been around for years, but is increasingly coming into question as it may be inaccurate. In a way similar to forensic hair analysis, retrograde extrapolation has even been called junk science by some. 

The idea behind the technique is that blood alcohol levels peak when the most alcohol has been absorbed into the bloodstream and the least amount of alcohol has dissipated from the system. Typically, the technique is employed in prosecution using toxicology experts.

Prosecutors using the technique try to provide their experts as much information about the defendant as possible, including the time of the offense, the time of the test, the result, the sex, weight, height, drinking history, number and size of drinks, the timing of the drinks, the alcohol concentration of the drinks, and food consumption. All of these factors are supposed to help the toxicology expert make a more accurate calculation. Among these factors, the most important are the alcohol consumption in the hour preceding the offense and any consumption after the offense and prior to the test.

In our next post, we’ll continue looking at this topic, and why it is important for those facing DUI charges in which blood alcohol content is estimated should always work with an experienced criminal defense attorney.

Sources:

The Gadsden Times, “How drunk? Blood-alcohol level estimates eyed in drunken-driving cases,” Frank Eltman, October 4, 2015.

American Prosecutors Research Institute, “Alcohol Toxicology for Prosecutors,” 2003. 

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