Retrograde Extrapolation

California DUI laws do not criminalize being under the influence or over .08% blood alcohol in a police station when being tested on a breathalyzer. The offense consists of being under the influence (DUI) or at least .08% blood alcohol when driving. Thus, the chemical evidence obtained form a breath or blood test at the station depends upon the ability to project the blood alcohol level backwards in time to when the DUI suspect was actually driving. This projection backwards in time, usually done by an expert witness from the prosecution’s crime lab, is called retrograde extrapolation. Put simply, it is the attempt of the prosecutor’s lab technician to guess what the blood alcohol concentration was of a DUI defendant an hour or so earlier.

In almost all Los Angeles DUI cases, there are serious flaws in retrograde extrapolation – flaws which must be made apparent to a jury in a DUI trial. The extrapolation process depends on a number of premises, many of which are subject to criticism. Thus, for example, it is assumed that the absorption of alcohol was completed, that the elimination of alcohol in the DUI suspect follows a uniform and predictable rate, and that the individual’s blood-alcohol “curve” may be accurately charted. Each of these assumptions in any Los Angeles DUI case, as will be seen, is fallacious.

The following comments from the noted blood-alcohol expert, Dr. Richard Jensen of Minneapolis, Minnesota, will give a general introduction to the dangers of retrograde extrapolation:

Retrograde extrapolation is the process whereby the result of a blood alcohol test taken sometime after an incident is used to determine the blood alcohol concentration at the time of the incident. In every case, there is insufficient information available to the expert to make this calculation. Assumptions are liable to he made which are not applicable to any specific individual. These assumptions would include that the alcohol was completely absorbed at the time of interest, the defendant eliminated alcohol at the average rate, and that the chemical test was proper and valid.

It has been demonstrated by continuous flow analysis that blood alcohol levels do not rise and fall in a steady fashion, but do demonstrate spikes or steeping where a momentary blood alcohol elimination is 0.015 percent per hour; this can vary in the general population from 0.008 to 0.030 percent per hour, and may vary even more than that in certain individuals. Any extrapolation for more than several hours will make the range so broad as to be almost useless. In order for retrograde extrapolation to be made, it is important to know that the individual has essentially absorbed all the alcohol he consumed, and his blood alcohol is now decreasing.

The rate of absorption is an unpredictable variable and will take anywhere from fifteen minutes to three hours after the last drink is consumed. There is some indication that this can be drawn out for up to six hours after the last drink is consumed under very extenuating circumstances. Any expert attempting a retrograde extrapolation should explain in detail the assumptions and the uncertainty involved. A specific individual could vary so greatly as to essentially make the extrapolation without value.

One variable in extrapolating blood-alcohol levels in a Los Angeles DUI case, the rate of absorption of alcohol, may be affected by a number of factors, such as the presence or absence of ingested food – and may vary further in relation to the amount and type of food consumed and when it was consumed. In an article entitled The Effect of Desmethylimipramine on the Absorption of Alcohol and Paracetamol, 52 Postgraduate Medical journal 139 (Mar. 1976), a group of scientists acknowledged that alcohol absorption “varies from person to person and can be changed by food, posture, and disease. It can also be altered by a number of different drugs, particularly those which modify the actions of the autonomic nervous system.”

Los Angeles DUI Laws and Lawyers

Los Angeles DUI Laws and Lawyers provides a resource for those accused
of drunk driving. It is also intended as an answer to those organizations working for ever more severe laws, greater use of unfair evidence and procedures, increasing destruction of our constitutional rights, and a new age of prohibition. See the National Motorists Association’s website,
which offers alternative commentary on issues such as DUI roadblocks, inappropriate criminal penalties and license revocations/suspensions imposed by the officer in the field.


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