The horizontal gaze nystagmus test, more commonly known as the HGN test, is among the three standard field sobriety tests (FST) conducted by law enforcement officials to gauge whether or not a DWI suspect is under the influence of alcohol. Officials use this test to establish probable cause to arrest an individual for DWI.
In the HGN test conducted during a DWI investigation, the officer instructs the driver to follow a particular moving object to the left and to the right with their eyes. The officer takes note of which angle the driver’s pupil begins to exhibit nystagmus, or the jerking of the eye involuntarily when the eye gazes towards the side. A high blood alcohol concentration is generally associated with an onset of nystagmus at or before an angle of 45 degrees.
The HGN test is based on the known fact that the alcohol affects the eyes’ automatic tracking mechanisms, slowing down the eyes’ ability to quickly track objects and causing the eye to jerk faster than if the individual were sober. The higher the individual’s blood alcohol concentration the faster the eyes will jerk when moving from side to side. The individual experiencing the nystagmus is commonly oblivious of its occurrence.
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.