“Retrograde extrapolation” is a pretty name for trying to guess backwards.
San Diego DUI attorney prosecutors in a San Diego DUI trial offer BAC test evidence guessing one’s BAC…back to the time of driving.
A blood-alcohol level at the time of a San Diego DUI chemical test is not relevant to the San Diego DUI charge.
San Diego DUI prosecutors therefore attempt to offer evidence to show what the BAC was when the San Diego DUI accused person was actually driving.
This is commonly done by “extrapolating” backward or computing the earlier blood-alcohol level by estimating how much alcohol had been eliminated or “burned off” in the period between San Diego driving and DUI testing.
But retrograde extrapolation requires two assumptions:
(1) The San Diego DUI accused person’s blood-alcohol level was declining; and
(2) The San Diego DUI accused person’s the rate of elimination is known.
Second assumption further involves the San Diego DUI prosecution lab employee’s assumption that the “burn-off” rate was .015% per hour (sometimes the assumed rate is .02% per hour).
Over a San Diego DUI Criminal Defense Attorney‘s objection, how does the San Diego DUI prosecution know that the San Diego DUI accused person 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?!
Sadly the San Diego DUI attorney prosecutor really does not know.
A San Diego DUI prosecution laboratory employee merely assumes that the San Diego DUI accused person 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. There’s been lots of studies.
“Indeed, “retrograde extrapolation” is nothing more than the prosecutorial version of the ” ‘rising blood-alcohol’ defense.” (People v. Beltran (2007) 157 Cal.App.4th 235, 246 [68 Cal. Rptr. 3d 489].) Each starts with the defendant’s blood-alcohol level at the time of the chemical test and relies on circumstantial evidence regarding the direction of change to convince the trier of fact that the level was different–significantly higher or lower–at the time of driving.” [PEOPLE, v. WARLICK, 162 Cal. App. 4th Supp. 1, fn 2; 77 Cal. Rptr. 3d 564; 2008 Cal. App. LEXIS 740]
This quote from Beltran is wrong. Here’s why:
When San Diego DUI criminal defense attorneys raise the issue of a rising blood alcohol defense, they do not try to connect the estimated driving BAC at a given time to a two decimal point number. That is what San Diego DUI prosecution experts do all the time, and when they do so they leave science behind and enter a world of pure guesswork and speculation, one dependent on the arbitrary assumptions they select for, and include in, their calculations.
A San Diego DUI defense rising BAC argument, on the other hand, is quite different and is based on certain alcohol consumption, food and other foundational facts presented to a San Diego DUI defense expert. These facts typically not available to San Diego DUI prosecution experts unless and until a San Diego DUI defendant testifies, or unless others who were present offer the facts.
San Diego DUI defendants argue that the probability is, based on the available facts, and in particular on the eating and drinking history of the driver and on the activities prior to the San Diego DUI police contact, that the BAC in all likelihood still rising at the time of the actual driving. Thus, the alcohol level was most likely lower than the test value obtained an hour or so after the San Diego DUI police contact, and in any event, the later test does not represent the accused’s BAC at the time of driving.
When this is used by San Diego DUI defendants, it must be limited to general propositions because San Diego DUI defense experts, like San Diego DUI prosecution experts, cannot scientifically and with real accuracy reconstruct one’s actual BAC curve that happened on the day in question.
Based on all the variables involved, it is improper for a San Diego DUI defense expert – and also with the San Diego DUI prosecution experts – to try to assign a given number to the driving BAC except in the broadest terms: the accused’s BAC at time of driving, based on the facts, was probably lower than the later test’s number. Naturally, the more time between the San Diego DUI police contact and that later test, the more uncertain is the actual driving time value, and the lower the actual BAC at time of driving may have been.
This San Diego DUI defense is very different than the practice of San Diego DUI prosecution experts who use retrograde extrapolation to give an opinion, for example, that based on a reported .06% BAC 80 minutes after driving, that the BAC at time of driving was in fact .08%!
A number of San Diego DUI prosecution experts believe they can try to raise this insupportable estimation or guess to the level of genuine science by adding a range to the opinion. Such a witness might say, “the driver’s BAC at time of driving was most probably between 0.07% and 0.09%, or more.”
Because the expanded actual range of possible values is so much larger than that opinion, adding that range to the opinion does not move it into the realm of legitimate science. The San Diego DUI prosecution expert’s opinion really never transcends speculation or guesswork.
This is why San Diego DUI prosecution “expert” retrograde opinions are junk science. However, a valid opinion in general terms, preferably based on actual, specific facts provided, that the BAC at time of driving might well have been lower than the BAC number reported at the time of the test 80 or so minutes earlier. Such a San Diego DUI opinion may be either right or wrong but that is the most that can be said.