Blood alcohol level (BAC) is a term used to describe the level of alcohol in the bloodstream of a person arrested for drunk driving. It is used in court as evidence of that offense. The most common method of determining BAC is through a breath test, although blood and urine testing is done in some instances. If the level is found to be at or over .10, or .08 in some states, the test results can establish a presumption of impairment.