Retrograde Extrapolation

Have you ever heard the term “retrograde extrapolation?” Wrapped in science and math, retrograde extrapolation is a process used by chemists and toxicologists to estimate an individual’s blood alcohol content at a specific time. It’s application in the context of DUI cases is often used to determine whether or not a driver had a BAC 0.08 or higher at the actual time of driving based on the time of testing.

The consumption of alcohol causes an individual’s blood alcohol level to rise as it gets absorbed into the system. The BAC level plateaus and falls as the body metabolizes and eliminates the alcohol.

An arrest on suspicion of DUI will consist of the accused undergoing chemical testing via a breath or blood sample. The refusal of chemical testing after a lawful DUI arrest is in itself illegal and a charge of DUI refusal.

As a result of a DUI refusal, drivers are subject to lose their driving privileges for a year. While each test may be challenged, both the breath and blood tests have been deemed reliable for providing accurate readings of subject’s BAC at the time of testing.

Retrograde extrapolation is commonly used by prosecutors to estimate a driver’s blood alcohol content at the time of driving. That being said, the use of retrograde extrapolation to estimate a defendant’s BAC at the time of driving as been criticized by many field experts.

The method requires toxicologists to make number assumptions. These assumptions include the assumption that the alcohol was completely absorbed at the time testing was conducted, that the rate of alcohol elimination occurred at the “average” rate and that the driver’s BAC curve can be charted with accuracy.

Alcohol absorption and elimination rates differ greatly in individuals. It can be affected by body temperature, the type of alcoholic beverage consumed, the amount of food consumed with alcohol and the weight and sex of the subject.

With so many factors affecting an individual’s BAC and the fact that retrograde extrapolation has come under the scrutiny of many experts who claim it’s assumptions present assumptions that could be used to determine if indeed the driver was guilty of a DUI.

It is not uncommon for defendants fighting a DUI charge to obtain an independent toxicologist’s testimony regarding retrograde extrapolation and its flaws. In addition, the independent toxicologist can rebut the State’s expert testimony regarding retrograde extrapolation.


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