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.
California'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, Morissette v. United States, 342 U.S. 246 (1952); and see McLean v. Moran, 963 F.2d 1306 (9th Cir. 1992), where the court held that Nevada's three-hour presumption was unconstitutionally applied as a mandatory presumption.
Thus the defense attorney must offer evidence to rebut this presumption — that is, evidence that the defendant's blood-alcohol level at the time he was driving was lower than the level at the time he was tested. This can be done, for example, by showing recent alcohol consumption, causing a rising blood alcohol curve.