This argument, commonly referred to as the "relation back" or "rising curve/BAC" defense argument, acknowledges that under Virginia law, the results of a breath test are rebuttable. In other words, if a person's BAC is .08 at the time of the test, Virginia law creates a rebuttable permissible inference/presumption that it was also a .08 at the time of driving. As mentioned, this inference/presumption is rebuttable.
Through the use of an expert witness, many times it is possible to present evidence that the accused's BAC was below the legal limit at the time of driving even though it was above the legal limit when they were tested two to three hours later. Read More.