Before a police officer can perform a test to determine a person's blood alcohol concentration, the officer must have a suspicion that the person may be intoxicated. Asking the driver to perform certain tests is the most common way for an officer to confirm that suspicion. While the BAC may tell the level of intoxication a person has reached, the Standard Field Sobriety Tests typically confirm that a person in Tennessee is intoxicated before any blood is drawn or Breathalyzer administered.
In the first of three tests, the officer observes the natural jerking motions in the driver's eyes to see if they are exaggerated or if the driver has difficulty following a moving object. Next, the driver is asked to take nine steps heel-to-toe in one direction before turning and walking in the opposite direction. An intoxicated person may have trouble following multiple directions and keeping his or her balance. The final test also marks the driver's balance by asking him or her to stand on one foot for 30 seconds without swaying or swinging the arms.
The challenges in the SFST are approved by the National Highway Traffic and Safety Administration. If the driver fails these three tests, the officer will ask the driver to submit to a Breathalyzer. The results of the BAC, along with the officer's observations from the SFST, will be offered in court as evidence that the driver was too impaired to be behind the wheel.
The observations of field sobriety tests are accepted in court as proof of drunk driving 94 percent of the time when a positive BAC result is ruled false. This is why contesting the BAC may be only one component of a person's defense if arrested for drunk driving in Tennessee. Having a skilled defense attorney will improve one's chances of avoiding the many negative consequences that may follow a DUI arrest.
Source: FindLaw, "Field Sobriety Tests", Accessed on Dec. 18, 2016