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How accurate are after-the-fact blood alcohol measurements?

On Aug. 30, 2015 this blog wrote about a collision on the Southern State Parkway in which three people were killed. The driver who caused the accident was arrested for DWI about one hour after the accident. A friend had driven him away from the scene. He was subjected to a test of his blood alcohol content level that gave a result of 0.06 percent, below the legal limit of 0.08 percent. Nevertheless, he was charged with drunken driving and vehicular homicide because a forensic technique known as “retrograde extrapolation” provided an estimated BAC level at the time of the accident of 0.12 percent.

This technique is now coming under frequent attack by defense attorneys who argue that the technique is unreliable. The attorney for the man charged in the Long Island accident says that “retrograde extrapolation is about as reliable as astrology.” According to this attorney, the technique is premised on an unprovable assumption that the defendant’s BAC peaked prior to the arrest.

The accuracy of a BAC level reading determined by retrograde extrapolation depends upon the rate at which a person’s blood absorbs alcohol and the rate at which it eliminates alcohol. Defense attorneys argue that these rates vary sharply from person to person depending upon gender, the kind of beverage consumed and whether the person had eaten food. For these reasons, they claim that the technique is not reliable. The uncertain reliability of this test is shown by the fact that most states leave the relevance of a retrograde extrapolation and its probative value to the jury to determine.

Challenging an after-the-fact measurement of BAC level is only one of the services that an experienced criminal defense attorney can provide for a client. Other benefits of consulting a lawyer who specializes in defending DUI cases include an evaluation of the circumstances, an enumeration of potential defense strategies, possible sentences including fines and license suspension and the likelihood of obtaining a favorable plea agreement or outright acquittal.

Source: Seattle Times, “How drunk? Estimates questioned in drunken-driving cases,” Frank Eltman, Oct. 4, 2015