The law requires that the state prove the suspect's alcohol concentration at the time of driving.  While your alcohol level at some point after the driving might be relevant to your alcohol level at the time of driving, the determining factor is whether you were over the legal limit (or impaired) at the time of driving.  Because in most cases a sample is not taken until at least one hour after the alleged driving, the result might not be indicative of the actual alcohol concentration at the time of driving. I have seen many situations where clients have consumed alcohol shortly before driving. 

Alcohol doesn't immediately absorb into your system.  It takes time.  Thus, in a situation where a person consumed alcohol shortly before driving, it is critical to determine how much of that alcohol was actually in their system when they were driving.  What is sometimes termed a "blood alcohol reconstruction" should be performed. In some cases, an attorney might hire an expert to do the reconstruction.  In other cases, the attorney might use a chart provided by the Wisconsin DOT to do the reconstruction. 

What is important is that the blood alcohol reconstruction be performed.  It is necessary to determine if the individual's alcohol level was on the rise, falling or peaking at the time of the driving.  If the alcohol level was rising, there might be a "curve" defense.  In a "curve" defense situation, the attorney argues that the alcohol test result does not accurately reflect the alcohol level at the time of driving.  The argument might be that while the alcohol test (taken an hour or so after the driving) shows an alcohol concentration over the legal limit it does not necessarily show the alcohol concentration at the time of driving.  This defense might be useful in a situation where the individual consumed alcohol shortly before driving. Because alcohol takes 30-90 minutes to fully absorb in the system, it might be plausible to argue that at the time of driving, the alcohol level was lower (under the legal limit) than at the time of the test.  The alcohol level at the time of driving would have been lower because of the unabsorbed alcohol (the drinks you had consumed minutes before driving) in the body.