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
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.).
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
- 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.
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)
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.