Retrograde extrapolation is a method for calculating what your blood alcohol concentration (BAC) was at an earlier time, based on the result of a BAC test taken at a later time. It is used in DUI cases where the test was delayed until hours after the person was pulled over.
How does retrograde extrapolation work?
After you drink alcohol, your BAC follows a pattern where first it rises and then falls. For a while after drinking, BAC continues to rise as your body absorbs more of the alcohol that was already consumed. Then, as your body eliminates the alcohol, the BAC falls again until you have no alcohol in your system. This process can take many hours, so BAC could still be rising or falling long after you’re done drinking. Retrograde extrapolation takes advantage of this slow curve by estimating BAC on an hour-by-hours basis based on later test results.
Is retrograde extrapolation accurate?
Not always. The formula relies on a standard rate at which the human body eliminates alcohol. Most people’s bodies follow this rate very closely, but not everyone’s. Additionally, the formula relies on the accuracy of the breath test or blood test itself, and both of these tests have margins of error and are susceptible to human error.
Why do police and prosecutors use retrograde extrapolation?
They use it primarily in cases where they expected you to have a BAC higher than the legal limit of .08% or above, but the test revealed that you were below that legal limit. In these circumstances, if the prosecutor really wants to try for a conviction, they will argue that your BAC would have been higher at the time you were driving, and had simply gone down by the time you were tested. They are trying to construct a case against you despite a lack of hard evidence.
Does it matter when I started and stopped drinking?
Yes. Retrograde extrapolation assumes that your BAC had already peaked and was coming down over the hours before the test. But there is also a phenomenon known as rising BAC. Basically, if you hd just finished drinking before you got in the car—especially if you had a full stomach—your BAC may have actually been going up, not down, by the time you were tested. This would suggest that you were actually less drunk, not more, when you were driving.
Can I really be convicted based on extrapolated BAC?
It’s hard to predict. On the one hand, extrapolated BAC is a weak basis for a DUI charge and your lawyer has an advantage when fighting this case for you. On the other hand, your case is far from a sure thing and people can and do get convicted based on extrapolated BAC. You need to speak to an experienced lawyer and take your charge seriously.
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