The law in Tennessee says a person is per se (presumed) intoxicated if the person’s blood alcohol content (BAC) is over .08.
If a person does not give blood or breath to determine the BAC, the officer is able to testify about his observations of the individual charged with DUI. These observations include: contact from the stop, slurred speech, blood shot eyes, odor of alcohol, field sobriety tests, appearance, and other indicators of impairment.
Impairment is not limited to alcohol. A person can be intoxicated by legal or illegal drugs. You may be prescribed medication by a doctor and later charged with a DUI if the officer believes you are under the influence.