Is a Retrograde Extrapolation in DUI Cases Reliable?

Any DUI attorney in California worth his salt knows that prosecutors will pull out a bag of tricks in order to get a conviction.  One of the most common arguments used by the DA is that of Retrograde Extrapolation.

In many DUI trials the district attorney will ask the state’s expert to perform a retrograde extrapolation from the defendant’s blood test results to determine the defendant’s blood alcohol at the time of driving. Although the prosecution gets the benefit of a "3 hour presumption" that the chemical test is a true reflection of the defendant's BAC if they were driving within 180 minutes of the test, most DA's will try and guild the lily by bringing in this often very prejudicial evidence.

However, this area of science is fraught with reliability issues.  The criminalist doing the “retrograde extrapolation” calculation must know, among other things, that the test subject is in the “eliminative” phase. Thus, a criminalist cannot do a valid “retrograde extrapolation” without a drinking and eating history.  There is scant scientific literature or case law that supports a “retrograde extrapolation” when the expert or person performing the calculation does not have any of the following factors: (1) when the person began to drink; (2) when the person stopped drinking; (3) how much the person drank; (4) what the person drank; (5) how much the person weighs; and (6) if the person had a full or empty stomach.

One of the top blood alcohol experts in the nation, Dr. Kurt M. Dubowski, summarized the problems with “retrograde extrapolation” in his frequently cited article, “Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects,” first published in the Journal of Studies of Alcohol.  In that study Dr. Dubowski concluded “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.   While cases dealing with this issue in California are few and far between, The Criminal Appeals Court of Texas in Mata v. State, 46 S.W.3d 902, addressed this issue and the scientific technique of “retrograde extrapolation” in excruciating detail. The Mata Court took judicial notice of scientific literature in the area and cited in its opinion numerous publications. The cited authority included that of Richard Watkins, Assistant Director of the Phoenix Crime Lab, and Eugene Adler, a toxicologist for the Arizona Department of Public Safety. Id. at 910; see “The Effect of Food on Alcohol Absorption and Elimination Patterns,” 38 J. of Forensic Science 285-291 (1993). The Mata Court, citing from Watkins and Adler, stated that: "The limitations and pitfalls associated with retrograde extrapolations are often not appreciated by lawmen and the courts. The authors [Watkins and Adler] conclude that “any attempt at retrograde extrapolations should be made with caution, and performed by a person able to assess and discuss the applicability of a retrograde extrapolation to a particular situation.” Id. at 910.

The court noted that Watkins and Adler were cautious about the reliability of retrograde extrapolation. Id. The court relying on other experts in the field wrote the following:  They [Watkins and Adler] write that retrograde extrapolation is a “dubious practice” and that expert testimony on the issue “requires careful consideration of the absorption kinetics of ethanol and the factors influencing this process.” They explain that “the absorption profile of ethanol differs widely among individuals, and the peak [BAC] and the time of its occurrence depends on numerous factors. Among other factors, the drinking pattern, the type of beverage consumed, the fed or fasted state, the nature and composition of foodstuff in the stomach, the anatomy of the gastrointestinal canal, and the mental state of the subject are considered to play a role.”

The Mata Court acknowledged that few jurisdictions have considered the reliability of “retrograde extrapolation” because many states have eliminated the need for “retrograde extrapolation” as a matter of law. Id. at 913. The statutes in these jurisdictions provide for a rebuttable presumption if the person’s BAC is over the legal limit, “assuming the test was conducted within a specified or reasonable time from driving.” Id.  The Mata Court was only able to find two courts in the entire nation that have touched upon issues of reliability of “retrograde extrapolation.” Id. An Arizona appellate court made reference to the issue in a footnote stating that “the science of ‘retrograde extrapolation’ has achieved general acceptance in the scientific field.” Id. citing Ring v. Taylor, 141 Ariz. 56, 59 fn 6. (Ariz. App. 1984).Thecourt that discussed the issue of “retrograde extrapolation” was the Alabama Court of Appeals in Smith v. Tuscaloosa, 601 So. 2d 1136 (Ala. Crim. App. 1992). Id. at 913-914. The Alabama Court disagreed with the Arizona Court and found from studies that “retrograde extrapolation” is an unreliable method of determining a persons BAC. Id.

The inadequacies of retrograde extrapolation extend beyond mere technical inaccuracies to problems that 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. Id. at 914.  After complete and thorough study of retrograde extrapolation, the Mata Court concluded that, even those who advocate retrograde extrapolation as a reliable technique, use 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.” Id. at 915. The Texas Court further concluded:  ​The court evaluating the reliability of a retrograde extrapolation should also consider (a) the length of time between the offense and the tests administered; (b) the number of tests given and the length of time between each test; and (c) whether, and if so, to what extent, any individual characteristics of the defendant were known to the expert in providing the extrapolation. These characteristics and behaviors might include, but are not limited to, the person’s weight and gender, the person’s typical drinking pattern and tolerance for alcohol, how much the person had to drink on the day or night in question, what the person drank, the duration of the drinking spree, the time of the last drink, and how much and what the person had to eat either before, during, or after the drinking. Id. at 916.