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It is common for a statutory BAC (blood alcohol content) level to determine whether the operator of a motor vehicle is guilty of Driving Under the Influence (DUI, which in some states is called DWI for "Driving While Intoxicated").

My question is: Do any states have a provision in law for interpolating a suspect's BAC backwards in time? It is known that alcohol is metabolized over time. I am wondering if a lower threshold on that metabolic rate has been codified anywhere in law.

E.g., suppose (as is the case in many jurisdictions) that the statutory BAC threshold for DUI is 0.08%: If a suspect is measured to have a BAC of 0.07%, but is known to have been driving at some point in time prior to the measurement, how (if at all) do any jurisdictions allow the suspect's BAC to be interpolated to the earlier point in time?

For example, there may be some significant delay between when a suspect is observed driving and when the suspect's BAC can be assessed with a legally sufficient test. During that time the suspect's BAC continues to decrease through alcohol metabolism. Given enough of a delay, a suspect who was in fact operating with BAC over the legal limit may end up showing a BAC under the legal limit when finally tested. Do any jurisdictions allow the state to account for that delay to assert that the BAC while operating the vehicle was higher than the level read by the test?

  • You'd also have to proof that the subject wasn't drinking between the drive and the test. To credibly extrapolate: you'd have to establish the exact drinking history: what was consumed when? – Hilmar Jan 25 at 19:54
  • The way metabolism works, BAC continues to increase for a significant time (1-2 hours?) after the last drink is consumed. So there's a fair bit of time where a person's BAC will be higher than when they were driving. Basically means that a breath-test will say one level and a later blood test will give approximately the same, if not delayed more than a few hours, but then such a delay would probably show a lack of desire to prosecute for it or defend against any potential charge – Nij Jan 25 at 20:54
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    @Hilmar It would not be in an uncontrolled environment but the scenario is exactly what happened to somebody I know: They were most likely DUI when pulled over, but he demanded a blood test. He was arrested but the police car got stuck in traffic and by the time they got to the hospital and the blood drawn, his BAC was under the limit. As a result, he could not be charged. – user71659 Jan 25 at 21:36
  • @Nij & Hilmar: Yes, BAC goes up as drinks are absorbed, and down as the alcohol is metabolized. So we know there are scenarios in which the tested BAC is higher than while previously driving, and we know that in those cases the tested BAC is what is used in any prosecution. Since drinking while driving is a separate offense, let us assume for the purposes of this question that the driver has not been drinking alcohol for some time before being observed driving, and therefore the (provable) reality is that the driver's BAC is strictly decreasing during the delay. – feetwet Jan 25 at 22:13

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