Typically when a person is charged with DWI, they submit to a breath or blood test soon after they have been driving so that the test indicates their blood alcohol content (BAC) at the time of their driving. However, what happens when there is a delay between the time the person is driving and the time of the breath or blood test? Because the body eliminates alcohol, a person’s BAC decreases over time. Therefore, it is possible that at the time of the defendant’s test the BAC can be below 0.08, but at the time of defendant’s driving the BAC would have been above. In this instance, the State will sometimes introduce the testimony of an expert in retrograde extrapolation.
The North Carolina Supreme Court has described retrograde extrapolation as
a mathematical analysis in which a known blood alcohol test result is used to determine what an individual's blood alcohol level would have been at a specified earlier time. The analysis determines the prior blood alcohol level on the bases of (1) the time elapsed between the occurrence of the specified earlier event (e.g., a vehicle crash) and the known blood test, and (2) the rate of elimination of alcohol from the subject's blood during the time between the event and the test.
State v. Cook (2008)
If the defendant is subjected to two tests sufficiently far enough apart in time, an expert could determine the defendant’s specific rate of elimination of alcohol. However, often the tests are not sufficiently far enough apart in time to make this determination and a testifying expert must use the average elimination rate of alcohol in his retrograde extrapolation.
The North Carolina Court of Appeals addressed the issue of using the average elimination rate in retrograde extrapolation in State v. Catoe (1985). In that case, the expert testified as to the defendant’s BAC at the time of the accident using the average rate of decline in BAC in his retrograde extrapolation. The defendant challenged the expert’s testimony based on “the requirement that the scientific technique on which the expert bases the proffered opinion be recognized as reliable.” The court of appeals concluded that the trial court did not abuse its discretion in admitting the expert’s testimony because he established “the general validity of his simple mathematical extrapolation” by determining the mean elimination rate and testifying “that the body eliminated alcohol essentially on a straight line basis.” The court determined that “[t]he possibility of minor variations conceded by [the expert] ... went to the weight, not the admissibility of his testimony.”
The court of appeals used the precedent in Catoe later in 2004 when they decided the case State v. Taylor. In Taylor, the defendant was involved in an accident around 1:00 pm. The law enforcement officer responded to the accident around 1:10 pm and noticed various signs of impairment. At that time, he subjected the defendant to various field sobriety test. However, the defendant did not take his first breath test until 3:18 pm, at which time his BAC was 0.05. The State’s expert performed a retrograde extrapolation using an average alcohol elimination rate and determined that the defendant’s BAC at the time of the accident was 0.08. The defendant challenged the expert’s testimony “because the elimination rate was based on an average, rather than defendant's specific rate” and therefore, “the conclusion of defendant's alcohol content level at the time of the collision was ‘without foundation, speculative, and mislead[ing][to] the jury[.]’”
The court looked to the North Carolina Supreme Court’s
three-step inquiry [set forth in Howerton v. Arai Helmet Ltd. (2004)] concerning the admissibility of expert testimony: "(1) Is the expert's proffered method of proof sufficiently reliable as an area for expert testimony? (2) Is the witness testifying at trial qualified as an expert in that area of testimony? (3) Is the expert's testimony relevant?"
In this case, the defendant did not challenge the expert’s qualifications or the relevance of the testimony, so the court examined only the first inquiry - the sufficient reliability of the proffered method of proof.
Because the court in Catoe had permitted the admission of retrograde extrapolation based on an average elimination rate, rather than the defendant’s specific rate, the court concluded that Catoe was “the type of ‘specific precedent’ ... which is meant to encourage a trial court to favor the admissibility of extrapolation evidence based on an average elimination rate.” The court reiterated the statement in Catoe that “the possibility of minor variations ‘went to the weight, not the admissibility of [the expert's] testimony.’”
The court in Taylor was also persuaded in allowing the admission of the expert’s retrograde extrapolation testimony because the expert used a “very conservative rate” which “tends to ‘favor the final result because it's going to give you a smaller number.’” If a low rate of elimination is used, then the extrapolated BAC result will be closer to the BAC indicated in the later test. However, if a higher rate of elimination is used, this assumes that the body has more quickly eliminated the alcohol and the defendant’s earlier BAC would have been higher.
The Taylor court also emphasized the North Carolina Supreme Court’s statement in Howerton that the assessment of the sufficient reliability of the proffered method of proof
does not, however, go so far as to require the expert's testimony to be proven conclusively reliable or indisputably valid before it can be admitted into evidence. . . . Therefore, once the trial court makes a preliminary determination that the scientific or technical area underlying a qualified expert's opinion is sufficiently reliable (and, of course, relevant), any lingering questions or controversy concerning the quality of the expert's conclusions go to the weight of the testimony rather than its admissibility.
Therefore, an expert’s testimony using an average elimination rate is sufficiently reliable to be admissible, but the possibility of a defendant’s individual elimination rate different goes to the weight of the testimony.
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