The Problem With Back-Calculating Blood Alcohol Content


“Extrapolation of a later alcohol test result, to the time of the alleged offense, is always of uncertain validity and therefore forensically unacceptable.”
-Kurt M. Dubowski, Ph.D.

The use of retrograde extrapolation (RE), or “back calculating” an individual’s blood alcohol concentration (BAC) in criminal matters, has many proponents and critics. Though prosecutors will often attempt to have RE in cases involving alcohol, some leading world scientists have strongly criticized the practice, noting there is rarely enough information and data available to backtrack an individual’s BAC to the time of the alleged offense.
RE is calculated by using the Widmark equation, which is most accurately done using multiple results from draws over a period of time, or by assuming the rate of elimination falls within an average rate of 0.010-0.025g ethyl alcohol/100mL blood per hour (Jones 2010). However, in order to accurately calculate RE, an individual must be in the post-absorption phase and their BAC curve must be linear.
The rate of alcohol absorption for an individual is heavily based on many factors such as food intake and the type and concentration of the alcoholic beverage. Sex, body weight, body water and other physical, biological and psychological factors also vary widely. Even in some of the most controlled experiments, elapsed time from end of alcohol intake to peak blood alcohol concentration can vary by more than two hours. The validity of calculating RE depends wholly on the accuracy and completeness of known variables, some of which might be identified in police reports and witness statements.
Kurt M. Dubowski, Ph.D., an expert in blood and breath alcohol research, outlined in his most cited scholarly writing, “Among the major reasons for the infeasibility of retrograde extrapolation, three stand out: (1) lack of knowledge, usually, about the timing of the alcohol concentration peak and absorption-post absorption status; (2) ignorance about the mathematical characteristics (e.g., linear, pseudolinear, exponential) and the mean rate of change of the individual’s blood or breath alcohol elimination curve; and (3) unpredictable irregularities of the curve, especially short-term fluctuations from the best-fit trend line of the blood or breath alcohol curve.”
The Colorado Bureau of Investigation (CBI) will not perform RE when a BAC result is less than 0.020 g/100mL, as this tends to be the level where blood ethanol stops decreasing at a constant rate. Once BAC stops dropping at a constant rate, ethanol is eliminated according to first-order kinetics and the rate of post-absorption may not be linear. However, at least one lab in the state has performed RE on a BAC result less than 0.010 g/100mL.
When reviewing a communication log report for RE, pay attention to notes on chain of custody. When blood kits are received by a forensic toxicology lab, they should be uniquely identified and labeled by the collecting officer with the subject name and/or agency case number, the collecting officer’s initials, and date and time of collection. Click here to view CBI’s specific protocol related to BAC Retrograde Extrapolation. Click here for a full list of laboratories certified to test blood as authorized by C.R.S. 42-4-13011304 and the Colorado Board of Health Rules and Regulations 5 CR 1005-2.

Rachel Roberts

Posted on July 25, 2017 .