During the administration of the horizontal gaze nystagmus test, an officer is generally supposed to watch for three clues or signs of intoxication per eye. The officer will then give out one point for every clue that is spotted per eye, which means that a total of six points could be given out throughout the entire test. If the DWI suspect has a total of four or more points, his BAC will be classified as above .10 percent.
According to the National Highway Transportation Safety Administration (NHTSA), the HGN test is believed to show an accuracy rating of 77 percent when it comes to determining if an individual has a blood alcohol concentration .10 or more. Experts, however, have criticized the reliability of this test due to several factors.
Officers who administer the HGN test are required to follow a detailed list of procedures outlined by the NHTSA. This list also includes clues to look for when scoring the test. Unfortunately, police officers are infamous for incorrectly administering this test – either due to improper training, lack of training, or failure to adhere to the procedures set. As a result, a number of innocent drivers are arrested due to improper testing methods and instructions.
While field sobriety tests such as the HGN test are often used to establish probable cause in a DWI, they are rarely accepted as proof of impairment.