The horizontal gaze nystagmus, or HGN, is one of three standard field sobriety tests promulgated by the National Highway Traffic Safety Administration. The other two standard tests are the walk and turn and the one leg stand.
The purpose of an HGN test is to determine if nystagmus is present in one or both eyes. Nystagmus is involuntary and automatic jerks that may result from the consumption of alcohol or certain other substances. Properly administered HGN can be an effective way to determine if a DUI suspect's blood alcohol content is equal or higher than 0.08.
In order for the HGN test to be considered as a valid indicator for the blood alcohol reading, at least four of six clues must be present. A clue is the term for what the officer is looking for when administering the test. There are a total of six possible clues, three per eye. The clues are as follows:
- 1) Lack of smooth pursuit (where the eyes have difficulty tracking a fast moving object)
- 2) Onset at less than 45 degrees
- 3) Prolonged nystagmus at maximum deviation
The test is conducted when an officer asks someone to look at an object, such as a pen, and then track it as it moves from side to side. Although everyone may have nystagmus at certain times, a failure to smoothly track a moving object, or jerking while looking only slightly to one side can be indicators of higher alcohol consumption. Nystagmus can be present with anyone when the eyes look as far as possible to one side, but if it doesn't stop after four seconds that can be a sign of alcohol consumption.
It is important to know that HGN does not test to see if someone is impaired or can't safely operate a vehicle; it only can indicate whether or not someone's blood alcohol concentration is likely to be at a certain threshold. Even under the best of circumstances it isn't foolproof one way or the other. Improperly administered the test is extremely unreliable.