Blood Alcohol Presumption
The majority of states have enacted statutes which "relate back'' the blood-alcohol concentration at the time of testing to the BAC at the time of driving. In other words, the blood-alcohol level at the time the test was administered is presumed to be the same as the level at the time the defendant was driving. Thus the legislature have given the prosecution yet another valuable weapon: retrograde extrapolation as a matter of law.
Ohio's statute is typical: "In any prosecution under this subdivision, it is a rebuttable presumption that the person had 0.08 percent or more, by weight, of alcohol in his or her blood at the time of driving of the vehicle if the person had 0.08 percent or more, by weight, of alcohol in his or her blood at the time of performance of a chemical test within three hours after the driving."
The presumption is rebuttable. This means that you have the right to present evidence that shows that you were not under the influence at the time that you were driving. If you have witnesses or other evidence that can establish that you consumed alcohol recently and the alcohol could not have been in your system at the time that you were driving, that evidence may be enough to establish a reasonable doubt as to your intaoxication at the time that you were driving.
The presumption that your blood alcohol content was the same at the time that you were driving as it is at the time of the test is very difficult to overcome. If you think that you have an argument that can overcome that presumption, you will need to have an attorney raise that issue before a trial. The attorney may be able to convince the prosecutor to reduce your charges or reduce the amount of your fine, but don't count on getting the charges dismissed.