Retrograde Extrapolation in DUI Cases: Voodoo Science

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Retrograde Extrapolation is a voodoo science being used by prosecutors all over the country to convict drunk drivers.

Many drivers mistakenly believe that if their blood-alcohol concentration (BAC) or breath alcohol content (BrAC) is below the .08 legal limit at the time they give a blood or breath sample, it is virtually impossible for the prosecution to secure a per se conviction. That is not necessarily true.

While it is true that under Michigan law (MCL 257.625a(6)(a)) the amount of alcohol in a driver’s blood or breath at the time alleged as shown by chemical analysis is admissible in any criminal proceeding and is to be presumed to be the same as at the time the person operated the vehicle, that is not the end of the story.

At trial, some state and city prosecutors still may attempt to use retrograde extrapolation to persuade the trier of fact that the BAC or BrAC exceeded .08 at the time the police initiated the traffic stop. But the truth is, the concept of retrograde extrapolation is based upon flawed science.

What is Retrograde Extrapolation?

The US National Library of Medicine states defines retrograde extrapolation as follows:

Retrograde extrapolation is a mathematical process, based on sound scientific principles, that is used routinely in pharmacology, toxicology, and clinical medicine. This process may be applied to the situation of ethyl alcohol consumption with reliability when reasonable assumptions are made concerning absorption rates, elimination rates, and patterns of alcohol consumption, including drinking duration and volume consumed. By utilizing an established range of values for the elimination rate of alcohol of 0.015-0.020 g/dl/h, a relatively narrow range of extrapolated blood alcohol concentrations (BACs) can be determined in situations where the time frame in question is after peak alcohol absorption into the blood. A wider range of elimination rates of 0.01-0.03 g/dl/h may be applied and will satisfy the possibility of nonlinear kinetics within an individual; however, this wider range will have little practical effect on the predicted BACs. When the time point in question is prior to peak absorption, a wider range of predicted BAC values will result. The extent of this range will be influenced by the amount of information available concerning the temporal pattern of alcohol consumption. Reported drinking volumes are notoriously inaccurate and, in fact, are of little practical use. Given the parameters of body weight and time duration between initiation of drinking and determination of the BAC, the number of “drinks” consumed may be reliability calculated. Retrograde extrapolation is applicable in the forensic setting with scientific reliability when reasonable and justifiable assumptions are utilized.

Retrograde Extrapolation: Unreliable for BACs

Retrograde extrapolation is based on the notion that a blood or breath alcohol concentration of an individual’s blood or breath sample at the time of testing is susceptible of being extrapolated back to a purportedly higher concentration at an earlier time; to wit: the time of driving. The calculation is achieved by augmenting the BAC or BrAC at the time of testing by the product of a specific ethanol elimination rate per hour and the time differential (number of hours) between the time of the stop and the time of testing.

The most significant flaw in the calculation is the application of a specific elimination rate. Many studies have shown, on average, that individuals will eliminate alcohol at the approximate rate of .015% per hour. In simple terms, that means that if a person consumes four alcoholic drinks that elevate his or her BAC to .08%, on average it will take approximately four hours for the alcohol to be completely eliminated from their system.

However, just as in the case of the partition ratio the elimination rate will vary from individual to individual. Indeed, without knowing the specific driver’s personal BAC profile raises serious questions about the efficacy of retrograde extrapolation in Michigan OWI prosecutions.

For example, a serious issue that must be addressed in any retrograde extrapolation exercise is whether the subject, at the time of the testing, was in the absorptive or post absorptive state.

Most state experts employed by the prosecution base their trial testimony on the fundamentally flawed assumption that the defendant was in the postabsorptive phase at the time of the testing. Moreover, they opine, without any credible factual basis, that the defendant’s BAC/BrAC profile in the postabsorptive state exhibits linearity. Indeed, most studies show that every BAC/BrAC elimination profile is susceptible to unpredictable irregularities and short term fluctuations. Consequently, many renown scientists in the field such as A.W. Jones has described retrograde extrapolation as “a dubious practice, owing to the many variables and unknowns involved.”

As noted by K.M. Dubowski in his seminal work Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects: “[n]o 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 … [and, furthermore,] extrapolation of a later alcohol test result to the time of the alleged offense is always of uncertain validity and, therefore, forensically unacceptable.”

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