Retrograde Extrapolation
by: M saltzman • June 15, 2010 • no comments
Retrograde extrapolation is the mathematical process of calculating a defendant's BAC at a time prior to the administered breath test. If a defendant is pulled over but is not given a breath test for some time, may the prosecution use retrograde extrapolation to establish that her blood alcohol content (BAC) was over the legal limit at the time she was driving? This inquiry poses two distinct questions:
===1. Can the state move forward on the .08 prong of a DUII charge without BAC test results of .08 or higher?
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Probably not. To succeed on a DUII charge under ORS 813.010, the state must prove either that the defendant was driving with a BAC of .08 or higher or that the defendant was driving while his behavior was perceptibly impaired. State v. King, 316 Or. 437, 446 (1993). To advance on the BAC prong, the state must show "by chemical analysis" that the defendant's BAC was .08 or higher. ORS 813.010(a). Where the relevant breath test results in a BAC level of less than .08, the state has not met its burden of proof as to this prong and the jury may not convict the defendant based on his BAC. See State v. Johnson, 219 Or. App. 200, 206 (2008) ("The jury was not permitted to convict defendant based on his BAC, given that it was not proved by chemical analysis to be at least .08 percent."); State v. Ross, 147 Or. App. 634, 640 (1997) ("The statute establishes what the state must prove to demonstrate a prima facie violation and that is by evidence from chemical tests only.").
===2. Is retrograde extrapolation scientifically valid under Brown and O'Key?
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Probably not. Evidence of a scientific nature may be highly persuasive to lay jurors, so a court must carefully assess whether such evidence is scientifically valid before allowing its admission. State v. O'Key, 321 Or. 285, 291 (1995) ("The function of the court is to ensure that the persuasive power is legitimate."). The proponent of the scientific evidence has the burden of establishing its validity. State v. Perry, 347 Or. 110, 122 (2009). Retrograde extrapolation is a questionable mathematical process whereby the results of a breath test are used to calculate the defendant's BAC at an earlier time. See Kurt M. Dubowksi, "Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects," 10 J. Stud. Alcohol Suppl. 98, 106 (1985). This process is based on two assumptions: (1) that at the time of the breath test the defendant was in the eliminative phase of his intoxication, when his BAC level was decreasing, and (2) that the defendant's blood was eliminating alcohol at a specific rate. However, "'the absorption profile of ethanol differs widely among individuals.'" Mata v. State, 46 S.W.3d 902, 910 (Tex. Crim. App. 2001) (quoting Richard Watkins and Eugene Adler, "The Effect of Food on Alcohol Absorption and Elimination Patterns," 38 J. of Forensic Science 285 (1993)). To properly determine whether the defendant was in the eliminative phase of intoxication and at what rate the defendant's blood was eliminating alcohol, the criminalist conducting the retrograde extrapolation must take into account several factors, including (a) when the defendant began to drink, (b) when the defendant stopped drinking, (c) how much the defendant drank, (d) what type of alcohol the defendant drank, (e) what the defendant weighs, and (f) if the defendant had a full or an empty stomach. See Mata 46 S.W.3d at 916; Dubowski, supra at 106 ("no forensically valid forward or backward extrapolation of blood or breath alcohol concentrations is ordinarily possible in a given subject and occasion solely on the basis of time and individual analysis results"). As such, without consideration of these factors, retrograde extrapolation should not be considered scientifically valid. See Mata 46 S.W.3d at 915 ("even those who believe retrograde extrapolation is a reliable technique have utilized it only if certain factors are known, such as the length of the drinking spree, the time of the last drink, and the person's weight"); Smith v. Tuscaloosa, 601 So. 2d 1136, 1140 (Ala. Crim. App. 1992) ("'The inadequacies of retrograde extrapolation extend beyond mere technical inaccuracies to problems which are inherent in the basic premises and calculations of this technique. These inadequacies render retrograde extrapolation inherently untrustworthy and therefore inappropriate for use as evidence to convict drunk drivers.'") (quoting E. Abbott, "One for the Road"-The Reliability of Retrograde Extrapolation and the Implications for Vermont Statutes, 16 VT.L.Rev. 395, 397 (1991)).
