What is the third clue for Horizontal Gaze Nystagmus (HGN)?

The third clue for Horizontal Gaze Nystagmus (HGN) is the onset of nystagmus prior to 45 degrees. This is significant in the context of evaluating a driver's impairment due to alcohol consumption. The presence of this nystagmus indicates that the individual's gaze begins to show signs of involuntary jerking when the eye is at an angle less than 45 degrees from the center.

In healthy individuals, nystagmus typically starts after reaching that 45-degree angle; thus, if an officer observes nystagmus occurring before reaching this threshold, it suggests a potential impairment due to intoxication. This is considered to be a reliable indicator, and its identification can be pivotal in evaluating a driver's fitness to operate a vehicle safely.