If you've had even one drink and you're asked to blow, you could register a warn, even though your BAC level is below 0.05. That's because you could have residual alcohol in your mouth. I have a friend with a breathalyser. A group of us were having some drinks over the course of an evening and we wanted to see how impaired we got. What we found was that downing half a glass of water before you blew, made a significant difference in the reading. Especially if you're sipping scotch or another spirit. At one point I blew 0.11, then after drinking water, blew 0.04. What also helped was hyperventilating.
Now of course, an officer will not let you drink anything before you blow. They'll also be suspicious if you're drinking water when they tell you to roll down your window. I also remember a cop friend tell me he's pulled over people who are clearly drunk, but before they get out of the car, they open a bottle of alcohol and take a sip. He said if someone does that, the breathalyser test is invalid. I can't remember how that plays out and what they can charge you with, but I don't think you can be charged with over 80. I'll have to talk to my cop friend again.