Criminal investigators and forensic scientists use a technique called retrograde extrapolation to estimate blood alcohol levels in individuals facing drunken driving or vehicular homicide charges. This technique has been used for years and is responsible for countless DUI convictions. However, this forensic technique has come under fire in recent months for its unreliable approximation of a person’s intoxication.

The basic idea behind retrograde extrapolation is that an individual’s blood alcohol level likely peaked at some point before they were arrested. Without any scientific basis behind this notion, many defense attorneys are demanding a limit to its use in trial.

There is no statistic offering a national number of convictions based on retrograde extrapolation. However, New York as well as several other states have used this type of forensic folklore for numerous DUI convictions. Courts in many other states have restricted retrograde extrapolation estimates in trial, basing DUI convictions solely on the BAC test reading taken at time of the arrest.

Blood alcohol levels vary widely between genders, drink of choice and experience. It is not uncommon for a BAC reading to inaccurately represent how intoxicated a person actually is. A persons BAC is highest when the highest level of alcohol has been taken into the bloodstream, before they have eliminated any. In retrograde extrapolation, the estimate becomes less accurate the further away it is tested from the last act of drinking. In most cases the time of the last drink is taken by word of the accused. If their estimates are off, so may be the retrograde extrapolation estimate.

Even the most respected and commonly used form of BAC testing has its faults. If you face a DUI charge based on a BAC reading or retrograde extrapolation reading you feel may be inaccurate, you may benefit from speaking to a trusted DUI defense attorney.

Source: Oneida Daily Dispatch New York, “Retrograde extrapolation’ estimates questioned in DWI cases,” Frank Eltman, Oct. 5, 2015