Drunk driving charges in Oklahoma are based on blood alcohol concentration, or BAC. Different levels of alcohol in the blood have been shown to affect driving differently, and the legal limit established across the country is currently set at .08 percent. Breathalyzer tests are used by police to measure BAC by the side of the road. Since the differences between a legal BAC and an illegal one can be small, it is important that these machines are properly calibrated. If they are not, people charged with DUI can raise that failure to maintain the equipment as part of their defense.