You can be charged with driving under the influence (DUI) even if your blood alcohol concentration (BAC) is as low as

Even if you haven't consumed any alcohol, your driving may be impaired by other drugs you've used. Therefore, even if your BAC is zero, you can be charged with DUI if a law enforcement officer has obtained other evidence that your use of certain drugs has made you incapable of operating a motor vehicle safely. [Driving Under the Influence of Drugs or Alcohol (The "DUI" Law); B-7. Alcohol, Other Drugs and Driving; Tennessee Comprehensive Driver License Manual]