Why Is Reverse Extrapolation of One’s BAC Junk Science?
Summary in 50 Words or Less: Reverse extrapolation of one’s blood alcohol content to the time of driving assumes that the driver metabolizes ethanol at a certain rate (“absorption rate”), 0.015% per hour, based on an average person’s age, gender, kidney function, liver function, body fat, emotional state, hydration level, metabolism, hematocrit, breathing rate and many other “averages” that may or may not apply to the driver. The calculation also assumes the BAC was declining throughout, which may not have been true.
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.
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).