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State v. Green and Retrograde Extrapolation

We have previously discussed the use of retrograde extrapolation in DWI cases in North Carolina. The North Carolina Supreme Court described retrograde extrapolation as follows:

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 a defendant submits to two blood alcohol tests that are spaced sufficiently apart in time, his specific rate of elimination of alcohol can be determined. In most cases, however, the tests are not sufficiently apart in time and a standard rate of elimination is used in the retrograde extrapolation analysis.

In 2010, the North Carolina Court of Appeals issued an opinion in the case State v. Davis, in which it held that retrograde extrapolation could not be used to determine a defendant’s blood alcohol level based solely on the odor of alcohol. After the court issued the Davis opinion, some theorized that this was the beginning of the end for retrograde extrapolation in north carolina. Courts in other states had already recognized some of the methodology’s flaws, such as the accuracy of a standard rate of elimination and the effect of food in the stomach on the rate of alcohol absorption.

However, in 2011, the court of appeals upheld the use of retrograde extrapolation based on a single BAC result from the defendant in the case State v. Green. In Green, the defendant caused another car to crash into a stop sign. The defendant did not stop until a witness to the accident stopped him. The witness described the defendant as having his eyes “kind of half shut and glazed looking” and not “really processing what [the witness] was saying to him.” The defendant did not stay at the scene of the accident and instead drove home. The accident was called into 9-1-1 at 8:06 p.m.

The officer investigating the accident went to the address to which the defendant’s car was registered at 9:38 p.m. When he arrived at the address, the officer saw a vehicle that matched the description of the defendant’s vehicle. During the conversation with the defendant, the officer noted that the defendant was “sluggish, slow” and had an “odor of mouthwash with a moderate to strong odor of alcohol coming through that.” The officer informed the defendant that he was investigating the accident. Although the defendant initially denied having anything to drink, he later claimed to have had one glass of wine. The defendant then changed his story to claim that he had five glasses of wine after arriving home at 7:15 p.m.

The defendant was arrested for dwi and given breath tests at 11:28 p.m., which indicated the defendant’s BAC was 0.19.

At trial the State used expert testimony involving retrograde extrapolation to determine the defendant’s BAC at the time of the accident. The expert assumed (1) the standard rate of elimination of alcohol, f(2) the accuracy of the defendant’s size and gender reported the intake sheet, and (3) 5 ounce glasses of wine containing 12% alcohol. The expert testified that, based on these assumptions, if the defendant had had nothing to drink after the accident, then the defendant’s BAC would have been 0.24 at the time of the accident. If the defendant had had one glass of wine after the accident, then the defendant’s BAC would have been 0.23 at the time of the accident. And if the defendant had had five glasses of wine after the accident, then the defendant’s BAC would have been 0.19 at the time of the accident. The expert noted that, based on these assumptions, if the defendant had nothing to drink before 8:06 p.m., he would have had to consume 88 ounces of wine after driving to register a BAC of 0.19 at 11:28 p.m.

On appeal, the defendant argued that the expert’s methodology was based on “impermissible factual assumptions,” specifically it assumed “the amount of wine in defendant’s glass and when it was consumed.” The court noted that for the trial court to determine whether “the expert's proffered method of proof [is] sufficiently reliable as an area for expert testimony” “requires a preliminary assessment of whether the reasoning or methodology underlying the testimony is sufficiently valid and whether that reasoning or methodology can be properly applied to the facts in issue.”

If the trial court does not have “precedential guidance” then it should look to other “‘indices of reliability’ to determine whether the expert's proffered scientific or technical method of proof is sufficiently reliable[,]” such as “the expert's use of established techniques, the expert's professional background in the field, the use of visual aids before the jury so that the jury is not asked ‘to sacrifice its independence by accepting [the] scientific hypotheses on faith,’ and independent research conducted by the expert.”

In this case, the court noted that the expert had testified in over 200 cases, more than 50% of those involving blood alcohol testing and more than 90% of the time involving blood alcohol physiology. In addition, the expert had conducted controlled drinking exercises on over 1,000 people during the last 12 years to measure blood alcohol concentration. Lastly, the expert noted that studies had been performed regarding people’s alcohol elimination rates over the last 70 years.

The court concluded that

Noting [the expert’s] use of retrograde extrapolation—a technique accepted in our courts since 1985, and his aforementioned training and experience, including independent research involving the analysis and measurement of blood alcohol concentration and elimination, and his challenge to opinion testimony about defendant's blood alcohol content at various times and under various scenarios based upon defendant's assertions of post-driving alcohol consumption went to the weight of the testimony to be determined by the jury rather than its admissibility so the trial court did not violate the parameters of Rules 702 and 703.

Therefore, despite post-Davis theorizing that the court might find fault with retrograde extrapolation, the court upheld the use of the methodology based on one BAC result and the standard rate of elimination of alcohol.

If you have been arrested for DWI, visit for legal help.

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