613.2Retrograde Extrapolation

Suggested Questions
Last Updated: 12/01/23

Background and Qualifications 

Mr./Ms. Witness, will you please tell the jury… 

1.  Name, background, basic introduction?

2.  Job title, location, description, years of experience?

3.  Previous employment, length of time?

4.  Education, training, other practical or personal study?

5.  Licenses, certifications, awards, honors?

6.  Publications, papers, presentations?

7.  Any other relevant qualifications/experience/background/curriculum vitae?

Tender the Witness as an Expert 

1.  Are you familiar with a principle known as “retrograde extrapolation”? 

2.  Can you please describe what “retrograde extrapolation” means?

3.  What formal training or study have you had regarding blood alcohol physiology and retrograde extrapolation, in particular?  

4.  When and where did you receive this training/study? 

5.  Have you ever previously been qualified as an expert in the field of blood alcohol physiology and pharmacology, including retrograde extrapolation? 

6.  If so, how many times, and in which courts?  

Tender the witness as an expert in “blood alcohol physiology and pharmacology, including retrograde extrapolation. 

Based on Reliable Principles and Methods 

1.  Can retrograde extrapolation be used to calculate what an individual’s blood alcohol concentration would have been at an earlier time? 

2.  What information do you need to know to in order to perform a retrograde extrapolation? 

3.  How is alcohol metabolized (i.e., eliminated) in the human body? 

4.  Do all people metabolize alcohol at the same rate? 

5.  Is there a typical or “standard” elimination rate that is generally accepted in the scientific community? 

6.  When you perform a retrograde extrapolation calculation, what elimination rate do you use? Why? 

7.  Please explain in more detail how retrograde extrapolation calculations are actually made? 

8.  Are the principles and methods you just described generally accepted and considered reliable in the field? 

Principles Applied to the Present Case 

1.  Were you asked to perform a retrograde extrapolation analysis in this case? 

2.  Did you prepare a report summarizing those calculations? 

[Provide witness with a copy, if neededMark and introduce the report into evidence.] 

3.  What information did you have about the defendant’s known blood alcohol concentration? 

[E.g., results of a blood test showing a BAC of 0.06, taken three hours after driving] 

4.  What other information did you need to perform a retrograde extrapolation in this case, and what did you use as a basis for that information? 

[Per State v. Babich, 252 N.C. App. 165 (2017), there must be “at least some facts” to support an assumption that the defendant had passed out of the absorptive state and entered into the elimination phase. The witness should explain any assumptions that were made in generating a “plausible timeline” for when the defendant entered a post-absorptive stateand the basis for making those assumptions (e.g., the defendant’s statements, the officer’s observations, or other circumstantial evidence such as a bar tab showing when the defendant purchased his last drink)Id. For more information, see the preceding entry on the law of Retrograde Extrapolation.

5.  Can you please walk us through the calculations you made in this case? 

[Follow-up as needed] 

6.  Based on those calculations, do you have an opinion, to a reasonable degree of scientific certainty, as to what the defendant’s blood alcohol concentration would have been at [earlier] time? 

7.  Based on your analysis in this case, would it be reasonable to believe the defendant could have had a BAC of less than 0.08 during (or at any relevant time after) the time of driving, and yet still have a BAC of [X] at the later time when it was measured? 

Why not?