Why Is Reverse Extrapolation of One’s BAC Junk Science?
This second assumption further involves the DUI prosecution lab employee’s (aka expert’s) assumption that the “burn-off” rate was .015 percent per hour (sometimes the assumed rate is .02 percent).
- The DUI arrestee’s blood-alcohol level was declining at the time of driving; and
- The DUI arrestee’s the rate of elimination is known.
Over the DUI defense attorney’s objection, how does the DUI prosecution know that the DUI arrestee was eliminating (assuming he or she was eliminating rather than still absorbing) at that rate and not at .005 percent, .3 percent or some other possible scientific rate?! Quite simply, the DUI prosecution does not know. The DUI prosecution laboratory employee merely assumes that the DUI arrestee was eliminating and that he or she eliminated at the average rate.
The problem is that everyone has a different metabolism, and even a given person will metabolize alcohol at different rates depending on many variables. These include one’s age, height, gender, weight, body fat percentage, hematocrit levels, kidney function, any liver abnormalities, what one drank, over what period of time, what one ate before and while drinking and even one’s mood (i.e. depression or happiness).
In one important study, researchers found a wide range of metabolism rates: some individuals can absorb alcohol and reach peak blood-alcohol levels ten times faster than others. (Kurt Dubowski, “Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects,” Journal on Studies of Alcohol (July 1985)).
As a result, scientists have concluded that the practice of estimating earlier BAC levels in DUI cases is highly inaccurate and should be discouraged. From the recognized expert in the field, Professor Dubowski of the University of Oklahoma:
It is unusual for enough reliable information to be available in a given case to permit a meaningful and fair value to be obtained by retrograde extrapolation. If attempted, it must be based on assumptions of uncertain validity, or the answer must be given in terms of a range of possible values so wide that it is rarely of any use. If retrograde extrapolation of a blood concentration is based on a breath analysis the difficulty is compounded.”
21 Journal of Forensic Sciences 9 (Jan. 1976); Reprinted in “Forensic Alcohol Supervisor Course,” California Criminalistics Institute – California Department of Justice, hosted by OC Crime Lab, 1998.
“[T]he practice of making back estimation of a person’s BAC is inevitably a controversial issue in DUI litigation and should be avoided whenever possible.” A.W. Jones & Barry K. Logan, Drug Abuse Handbook 2012, Reprinted in “Forensic Alcohol Supervisor Course,” California Criminalistics Institute – California Department of Justice, hosted by OC Crime Lab, 2000.
“Making back extrapolations of BAC is not recommended because of the wide variations in absorption, distribution, and elimination patterns of ethanol both within and between different individuals.” Id. at 347, citing: Allanowai, et. al., “Ethanol Kinetics – Extent of Error in Back Extrapolation Procedures.” 34 Br. J. Clin. Pharmacology 316 (1992); Lewis, “Back Calculation of Blood Alcohol Concentrations,” 295 Br. Med. J. 800 (1987).