“Retrograde extrapolation” in DUI prosecution, P.2

On Behalf of | Oct 22, 2015 | Drunk Driving

In our previous post, we began speaking about a technique sometimes used by prosecutors to estimate the blood alcohol content of a drunken driving defendant. As we mentioned, accurate retrograde extrapolation of blood alcohol content depends on a number of factors.

Retrograde extrapolation may also involve assumptions. Critics of the technique say that it relies on the assumption—though without any basis—that the suspect’s blood alcohol content reached a peak before the arrest. Because absorption and elimination rates can vary considerably depending on various factors, assuming regularity of these factors is not going to be accurate. The accuracy of the technique would also depend on when the suspect last had a drink, and typically this information has to either be obtained from the suspect or assumed. 

Different states have different rules regarding the acceptability of retrograde extrapolation as a form of evidence. Here in Texas, some judges interpret the law to mean that retrograde extrapolation is never admissible as evidence or that a blood test taken hours after the incident is inadmissible without extrapolation.

Because the state of the law isn’t settled with regard to retrograde extrapolation, it is important for defendants facing prosecution with the use of such evidence to work with an experienced attorney to make sure their rights are protected. Even if it isn’t possible in such cases for a defendant to have charges dismissed, it may at least be possible to minimize charges.


The Gadsden Times, “How drunk? Blood-alcohol level estimates eyed in drunken-driving cases,” Frank Eltman, October 4, 2015.

American Prosecutors Research Institute, “Alcohol Toxicology for Prosecutors,” 2003. 

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