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Driving Under the Influence of Alcohol (DUI): PK Legal Aspects
Consider the following example. You are a 140 pound healthy male who
is attending a party. You start drinking beer at 10 P.M. and
continue for about 30 minutes. You leave the party to drive home at
11:00 P.M. You are stopped by a police officer at 11:30 P.M, who
takes you to a police station for suspicion of DUI. At 2:00 A.M.,
you breath into a machine that measures your breath alcohol (via IR
spectroscopy), and a proprietary (unknown) algorithm converts your
breath alcohol concentration to blood alcohol concentration (BAC).
Your blood alcohol value turns out to be 0.06 grams per deciliter.
This is below the DUI threshold of 0.08 grams per deciliter. But the
question is this: what was your blood alcohol concentration at 11:30
P.M., when you were stopped driving by the police officer. In some
jurisdictions, a so-call "retrograde extrapolation" calculation is
utilized, employing the Widmark equation. In its pristine form, the
Widmark equation is:
C(t) = C(0) - K(t)
where t = time, C(t) is BAC at time t, C(0) is BAC at time zero, K is
the zero-order, fractional hourly decrease in BAC ("burn-off rate,"
with units of 0.017 grams per deciliter).
But in this case, the question is what was your BAC at the time the
police officer stopped you at 11:30 P.M., viz, 2.5 hours prior to
your BAC testing. Employing "retrograde extrapolation," the Widmark
equation is re-arranged as follows:
C(11:30 P.M.) = C(2 A.M.) + (0.170)(2.5 hours) =
C(11:30 A.M.) = 0.060 + (0.017)(2.5 hours) = 0.125, over the
threshold of 0.08. So, you are arrested, finger-printed, and sent to
Keep in mind, this could happen to you.
A good DUI attorney and consultant could find at least 20 ways to
attack the DUI arrest. From a PK standpoint, where would you find
all the potential errors (rhetorical question)?
Have yourself a "field-day," especially Nick Holford who has
published on BAC PK.
Harold Boxenbaum, Ph.D.
14621 Settlers Landing Way
North Potomac, MD 20878-4305
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