A conviction of driving under the influence (DUI) is determined by a blood alcohol concentration of:

Driving with a blood alcohol concentration (BAC) of 0.08 percent is always illegal in Tennessee. Drivers under age 21 will face stiff penalties for having a BAC as low as 0.02 percent. It is possible to be convicted of driving under the influence even if your BAC is below the legal limit.
DMV Writen Test