Retrograde extrapolation is a scientific method used to estimate a person’s blood alcohol concentration (BAC) at a prior point in time—often at the time of an alleged offense—based on a later BAC measurement. This technique becomes especially relevant in DUI cases where there’s a delay between the time a person was driving and when their BAC is actually tested. Experts rely on known alcohol absorption and elimination rates to “work backwards” and estimate what the BAC would have been earlier.

“[Karen Read] had a blood alcohol content (BAC) of 93 milligrams per deciliter, or 0.093%. The legal BAC limit in Massachusetts (and Georgia) for driving is 0.08%.

Later, Nicholas Roberts, who at the time was a member of the Massachusetts Office of Alcohol Testing working with the Massachusetts State Police crime lab unit, took the stand.

He spoke about a type of “retrograde” testing that allows the lab to estimate what a person’s blood alcohol level would have been hours before an official blood test was taken.

Read’s blood was taken at 9:08 a.m. Using 12:45 a.m. as the estimated last drink Read consumed, Roberts estimated that her BAC would have previously been a minimum of 0.135% or a maximum of 0.292%.”

–CBS NEWS

In the recent Karen Read case, which drew national attention due to its complex facts and high-profile nature, retrograde extrapolation was used by the prosecution’s toxicology expert to support the theory that Read was intoxicated while driving. Specifically, the expert testified about how Read’s BAC, measured hours after the incident, could be calculated backward to suggest impairment at the time of the alleged event. This testimony played a role in the prosecution’s narrative, though its reliability was challenged by the defense.

 

How Retrograde Extrapolation Works in DUI Defense

From a DUI defense standpoint, retrograde extrapolation can be both a sword and a shield. Prosecutors often use it to argue that a driver’s BAC was over the legal limit at the time of driving—even if the test was administered well after the traffic stop or incident. However, defense attorneys are equally equipped to challenge the reliability of this method.

Key factors affecting retrograde extrapolation:

1. Timing: The longer the delay between the alleged driving and the BAC test, the more speculative the estimate becomes. There’s just too many variables.
2. Absorption Rate: Alcohol doesn’t enter the bloodstream immediately; food intake, type of alcohol, and body composition affect how fast it’s absorbed.
3. Elimination Rate: Most people eliminate alcohol at a rate of 0.015% to 0.020% BAC per hour, but this can vary widely.
4. Assumptions: Experts must assume the driver was post-absorptive (no more alcohol being absorbed) at the time of the test, which may not always be accurate.

A  defense attorney can challenge retrograde extrapolation on multiple grounds:

Individual Variation: The method uses general population averages, which may not apply to the defendant.

Incomplete Data: If there’s only one BAC reading and no information about the driver’s drinking timeline, any extrapolation is built on shaky assumptions.

Scientific Uncertainty: The scientific community acknowledges limitations and the potential for large margins of error, especially with unknown drinking patterns.

If you’re facing a DUI charge give us a call today and talk to one of our skilled DUI Defense Attorneys.