Why is it important to assess nystagmus angles in the HGN test?

Assessing nystagmus angles in the Horizontal Gaze Nystagmus (HGN) test is crucial for determining blood alcohol concentration because it is one of the key indicators of impairment due to alcohol. Nystagmus refers to the involuntary jerking of the eyes, which can become more pronounced with increased levels of blood alcohol. During the HGN test, the officer observes the eyes as they follow a stimulus across the visual field. Specific angles at which this jerking occurs are associated with different levels of impairment.

When the eyes exhibit nystagmus at certain angles, it suggests that the individual's blood alcohol content (BAC) is above a certain threshold, typically 0.08% or higher in many jurisdictions. This physiological response is a reliable indicator that the individual may be impaired and unable to operate a vehicle safely. Thus, the assessment of nystagmus angles directly correlates with estimating blood alcohol concentration, reinforcing its importance in the HGN test.