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Critics line up to challenge DWI/DUI forensic technique

Advocates of something called "retrograde extrapolation" (we'll define that in a moment) are enthralled with it, with one proponent saying that "it's been proven to be reliable."

The counter viewpoint of many critics from coast to coast?

Uh, not so enamored. One calls it "hogwash." Another, who is an expert in the field within which it applies, says, "I understand why people say it is junk science."

Here's retrograde extrapolation 101, as explained in a quick hypothetical.

Police officers stop you for suspected drunk driving, but don't get around to testing your blood-alcohol level for, say, four hours. At that time you clock in at a 0.06, which is far beneath the DUI/DWI threshold in North Carolina or anywhere else in the country.

Nonetheless, the cops want to charge you with DUI/DWI for an alleged offense that occurred hours earlier.

Presto. Retrograde extrapolation (we'll occasionally call it RE from here on for simplicity's sake) now enters the picture. RE is a forensic technique that looks backward to measure BAC levels. Quite simply, it extrapolates.

The "science" has been used in many states, with a recent article on retrograde extrapolation and drunk driving noting that RE-related evidence has been considered by juries in North Carolina.

As stated, its proponents -- unsurprisingly, and for the most part, law enforcement officials and prosecuting attorneys -- say that it is eminently reliable.

The naysaying camp -- large and varied -- scoffs at that, with one common rejoinder simply noting that alcohol absorption and breakdown in a human body varies widely from person to person, based on gender, drinking habits, the beverage consumed, whether a person has eaten or not and additional factors.

"I challenge it every time it comes up," says an author on a DWI/DUI textbook.

Based on all the criticisms of retrograde extrapolation, that seems like a sound defense strategy to pursue.

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