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Field sobriety test

What is the Horizontal Gaze Nystagmus Test

The Horizontal Gaze Nystagmus (HGN) test is a common field sobriety test used by law enforcement officers to determine if an individual is driving under the influence of alcohol or drugs. It involves the officer observing the involuntary jerking of a suspect’s eyes as they follow an object, typically a pen or small flashlight, held about 12-15 inches from the suspect’s face, and moved at a designated speed. The HGN test has been held to be a “Scientific Test” under Texas caselaw, which means it needs to be administered in the manner described in the National Highway Traffic Safety Administration “NHTSA” Standardized Field Sobriety Test Manual.

While the HGN test can sometimes be an indicator of intoxication, there are several factors that can impact the results. In this blog post, we will explore some of these factors and how they can affect the outcome of the HGN test.

Medical conditions

Certain medical conditions, such as brain injuries, inner ear disorders, and certain medications can cause nystagmus or jerking of the eyes. These conditions can cause false positives on the HGN test, leading to incorrect conclusions about a person’s level of intoxication.


As people age, their eyes may become less stable, which can result in nystagmus even when they are not under the influence of drugs or alcohol. Therefore, older individuals may show signs of nystagmus during the HGN test even when they are sober, leading to false positives.

Lighting conditions

The lighting conditions in which the HGN test is administered can also impact the results. Bright or flashing lights can cause nystagmus or make it more difficult for the officer to observe the eyes, leading to inaccurate conclusions.

Improper administration

The HGN test must be administered properly to be accurate. If the officer does not hold the object at the correct distance or angle, or if they move it too quickly or in an erratic manner, it can cause false positives or inaccuracies in the test results.

Other factors

Other factors such as fatigue, stress, and anxiety can also impact the results of the HGN test. These factors can cause a person’s eyes to jerk or move in an irregular manner, even if they are not under the influence of drugs or alcohol.

While the HGN test can be a useful tool for law enforcement officers to detect impaired drivers, it is important to recognize that the test is not foolproof. Several factors can impact the results of the HGN test, leading to false positives or inaccuracies. Therefore, it is important for officers to take these factors into account when administering the test and interpreting the results.

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