The Horizontal Gaze Nystagmus (HGN) Test: Is it Reliable?

DUIs and the "look to the left" test. Are HGN tests reliable?

You’ve seen it in the movies and on TV (and maybe in real-life as well). An officer suspects a driver has been drinking and has the driver step out of the car. The officer pulls out a penlight, moves the light in front of the driver’s eyes, about one foot from the face and asks the driver to look at the light as it moves.

How The HGN Test Works

It’s a test for high blood alcohol content (BAC) and is based on a quirk in human biology. An involuntary twitch (referred to as Nystagmus) occurs whenever a person looks sideways at an angle greater than 45 degrees (triggering peripheral vision). However, people with a high BAC twitch when the light is held at less than 45 degrees. The procedure is known as the Horizontal Gaze Nystagmus (HGN) Test. Here are some things to keep in mind about HGN testing:

  • It’s accurate if administered properly. The National Highway Traffic Safety Administration (NHTSA) has determined that the HGN is a reliable test if administered by a police officer who has had HGN training. (Unlike many other field sobriety tests -- closing your eyes and estimating when 30 seconds has passed, putting both index fingers on your nose, or counting backward from 100 by sevens – HGN testing has scientific support.).
  • Some states don’t admit HGN test results. Despite its confirmed reliability, some states don’t admit HGN test results into evidence. However, even in these states, HGN testing may be used as the basis for probable cause for a DUI arrest.
  • You don’t flunk or pass an HGN test. Unlike BAC levels, an HGN test is not a pass/flunk exam for high BAC. Instead, the police typically use HGN testing as a general indicator of possible impairment.
  • Two types of twitching. There are really two types of Nystagmus: (1) Alcohol Gaze Nystagmus (AGN) which is commonly used for HGN testing and in which the eyes move but not the head, and Positional Alcohol Nystagmus (PAN) in which the eyes and head both move.
  • What’s the officer looking for? In the HGN test, there are three clues for each eye. (1) Do your eyes move smoothly or jerk noticeably, (2) Do your eyes jerk when you have moved them to the side as far as possible, and (3) Does your eye starts to jerk before it has moved through a 45-degree angle.
  • The subject's eyes must be seen clearly. If the subject’s eyes can’t be seen clearly, the test will not be admissible, that is, the HGN test must be administered in a well lit area or by use of a flashlight to illuminate the subject's face
  • The subject should not be facing oncoming headlights. An HGN test may be dismissed if the subject is facing toward the blinking lights of a police cruiser or passing cars (which may cause an effect known as optokinetic nystagmus)
  • Subjects should remove glasses. Removing eyeglasses makes it easier for the officer to observe eye movement. However, glasses do not effect the HGN test results and nor do contact lenses (though hard contact lenses may pop out when the eye moves far to the side).

Refusing a BAC Test When Arrested for DUI

To learn about the refusal laws in a given state, choose from the list below.

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