Blood Alcohol Presumption
Most 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, of course, rebuttable: a conclusive presumption would conflict with the presumption of innocence. The defendant (you?) may offer evidence to rebut this presumption that is, evidence that your blood-alcohol level at the time you were driving was lower than the level at the time the was tested. This can be done, for example, by showing recent alcohol consumption, causing a rising BAC curve. If it is established that you drank more alcohol after you were out of the car but before the test, your BAC reading could be significantly higher.