Administering the HGN Test

Before conducting the horizontal gaze nystagmus (HGN) test, the officer is trained to look for conditions or reasons other than alcohol that could cause the driver to fail the test. The object to be used in the test – a penlight or a finger – is quickly waved across the subject’s vision to see that both eyes move in tandem, without signs of blindness or injury. S/he checks to see that the driver’s pupils are of equal size; inequality indicates injury.

Once the officer determines that to the best of his or her knowledge the driver does not have a medical condition that would affect the eyes, the officer conducts the HGN test. A penlight or even a finger is tracked in front of the driver’s eyes – about one foot distant –and the driver is asked to follow the light or finger with the eyes.

The examiner is looking for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking of the eyball, instead of smoothly tracking, is within 45 degrees of center.

If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.10 or greater.