Key Concepts

  • Retrograde extrapolation uses a defendant’s blood alcohol concentration (BAC) obtained at a later time to estimate what his or her BAC was at an earlier time, based on the average rate at which alcohol is eliminated from the body.
  • North Carolina courts have consistently accepted the science behind retrograde extrapolation as reliable, and have allowed properly qualified witnesses to give an opinion.
  • However, the expert’s opinion must be based on sufficient facts and data specific to the case – for example, the witness may not simply assume (without a sufficient basis) that the defendant was in the elimination phase at the time the BAC was measured.


“Retrograde extrapolation” refers to the process of taking a known blood alcohol concentration (“BAC”) from a given time, such as the results of a blood draw obtained two hours after defendant was arrested, and using that number to calculate what the defendant’s blood alcohol concentration was at an earlier time, such as two hours before the blood draw when the defendant was operating a vehicle. See generally “Retrograde Extrapolation of Blood Alcohol Data: Abstract,” MR Montgomery, Journal of Toxicology and Environmental Health, August 1992

When a person consumes alcohol, the concentration of alcohol in the person’s blood rises as more alcohol is absorbed. Once the person stops drinking, the alcohol concentration eventually peaks, and then declines at a constant rate as the person’s body begins to metabolize and eliminate the alcohol. By multiplying the “elimination rate” (that is, the rate at which the human body processes and removes the alcohol) by the number of hours since the person stopped consuming and absorbing the alcohol, and applying that calculation to the later-obtained blood alcohol concentration results, a properly qualified expert witness can make a reasonably accurate estimate as to what the person’s previous blood alcohol concentration was. 

Practice Pointer

Suggested reading 
For more detailed information on how retrograde extrapolation works, including a discussion of related issues such as alcohol concentration, absorption rates, elimination rates, and answers to common defenses, see Alcohol Toxicology for Prosecutors,” American Prosecutors Research Institute, July 2003. 

Reliability and Admissibility 

Numerous North Carolina appellate cases, decided both before and after the 2011 Daubert amendments to Rule 702, have accepted retrograde extrapolation as reliable, and have permitted a properly qualified expert witness to give an opinion regarding what the defendant’s blood alcohol concentration likely was at an earlier time. See, e.g., State v. Turbyfill, 243 N.C. App. 183 (2015) (finding retrograde extrapolation scientifically valid and reliable; witness was properly qualified to give opinion); State v. Green 209 N.C. App. 669 (2011) (witness testified as an expert in pharmacology and physiology, and gave a reliable opinion based on retrograde extrapolation that defendant’s BAC was 0.16 or more at time of offense); State v. Taylor, 165 N.C. App. 750 (2004) (state’s expert testified that the average elimination rate of alcohol was 0.0165 per hour, and using a retrograde extrapolation method, opined that the defendant’s blood alcohol concentration was 0.08 at the time of the accident; court ruled that the state’s expert was properly permitted to use the “average” elimination rate of alcohol, and was not required to determine defendant’s personal elimination rate); State v. Catoe, 78 N.C. App. 167 (1985). (“Dr. Ellis admitted that there could be deviation from the mean in individual cases, but that his data were very consistent across the various subcategories of the population. He testified that the body eliminated alcohol essentially on a straight line basis, establishing the general validity of his simple mathematical extrapolation. On this record, we conclude that Dr. Ellis' testimony was sufficiently reliable and the court did not abuse its discretion in admitting it.”). 

However, while the science itself has been broadly accepted as reliable in North Carolina, the state still must show that the retrograde extrapolation conducted in this particular case was performed correctly and that it was based on sufficient facts and data – if not, the testimony may be excluded or the case may be reversed on appeal. See, e.g., State v. Babich, 252 N.C. App. 165 (2017) (extrapolation opinion should not have been admitted where expert assumed that defendant was in “post-absorptive state” at time of BAC measurement, but presented no factual basis to support that assumption – harmless error in this case); State v. Davis, 208 N.C. App. 26 (2010) (reversible error to allow retrograde extrapolation testimony where witness assumed a baseline BAC of at least 0.02 based solely on a report of the smell of alcohol on defendant’s breath 10 hours after the accident, and extrapolated back to 0.18 at time of offense – court found insufficient evidence of reliability for equating an odor on the breath with a 0.02 reading, so it was an abuse of discretion to allow the opinion into evidence).  

Per Babich, the prosecutor must ensure that the witness can provide an adequate basis and explanation for any assumptions that factor into the opinion. If the expert’s calculations are based on an assumption that the defendant was in a “post-absorptive” or “elimination” state at the time his or her BAC was measured, there must “some underlying facts to support that assumption.” State v. Babich, 252 N.C. App. 165 (2017). Those underlying facts can come “from the defendant's own statements during the initial stop, from the arresting officer's observations, from other witnesses, or from circumstantial evidence that offers a plausible timeline for the defendant's consumption of alcohol.” Id. For example, the expert might be able to establish a timeline for when the defendant had his or her last drink based on the defendant’s admissions to the officer, a receipt from the bar showing when the defendant closed his tab, the higher BAC results of the PBT administered out on the road, the officer’s observations of defendant’s improving/deteriorating condition, or any other relevant details in the case. As long as there are “at least some facts” to support the assumption that the defendant had passed out of the absorptive state and entered into the elimination phase, “the issue then becomes one of weight and credibility” for the jury, rather than a question of admissibility for the judge. Id. 

Portions of this entry were excerpted from the North Carolina Superior Court Judges’ Benchbook, “Criminal Evidence: Expert Testimony,” Aug. 2017, Jessica Smith.