It isn't impossible at all to build a case against a >.8 reading at the station. Those machines are not infallible, and each state has a set of guidelines for the administration of the test. Many, many factors can determine whether these test results are even admissible. You have to consider whether there was probable cause to pull over the driver in the first place. If not, any subsequent evidence is inadmissible. You have to consider whether the test was administered properly by an officer certified to administer the test, whether the machine was properly calibrated, whether the observation period was met before administering the test, whether any mouth alcohol from belching or vomiting was in the mouth, etc, etc. I could really write a hundred pages on this, but I will spare you....lol.
Also, diabetes and other medical conditions can render the results unreliable. Diabetics tend to have ketones in their mouth, which will make the test read higher. I have never had the opportunity to use this defense before, and I have never seen a case from any state using it, but I would love to have had a case where I could argue that someone was on a low carb diet and in ketosis, thus affecting the readings. That would have been cool! I didn't have any clients in ketosis, though...lol.
Well, I probably have already bored the shit out of you guys with my ponificating, but suffice it to say that if you have a criminal defense lawyer who is worth their salt, they will ask you every detail of what you drank, who was present, what you ate, any medical conditions, the stop, the on scene investigation, the modes of field sobriety testing, the arrest, the detainment. He or she will get the calibration records for the machine and verify whether the officer was certified to administer the test. In our state they are also required to wait 20 minutes and observe you during that time before they administer it. They also have to say you have the right to refuse it. There are other disclaimers they have to administer, as well.