In a DUI case, officers use many tests to decide whether to arrest a person for DWI. Among them is the HGN test, here is how it is performed:
A Stimulus, such as a pen or pencil is held in front of the subject’s face, approximately 12 to 15 inches (30 to 38 cm) from the subject’s nose and slightly above eye level. This elevated eye position raised the upper lids and allowed the evaluator a better view of the eyes, but did not affect the results of the test. The subject was instructed to keep his or her head still and follow the stimulus with the eyes only. The subject’s left eye was observed first during each of the three component tests. Smooth pursuit was assessed by moving the stimulus to extreme left gaze and then to extreme right gaze at about 30 deg/sec. The test was repeated at least once for each eye. Nystagmus at maximum deviation was assessed by moving the stimulus first to extreme left gaze, then to extreme right gaze, such that no temporal sclera showed at either position, and held at each position for at least 4 seconds. Onset of gaze nystagmus was assessed by moving the stimulus at about 15 deg/sec to each side until nystagmus was observed. If nystagmus was present, the evaluator determined whether the angle of onset was less than 45 degrees.
According to one DUI Attorney, the HGN test is scored by the number of signs present for the two eyes, scoring one sign each per eye for lack of smooth pursuit, sustained nystagmus at maximum deviation, and onset of gaze nystagmus prior to 45 degrees. Therefore, the maximum number of signs is six. Previous laboratory and field validation studies have consistently demonstrated that the presence of four or more signs is highly correlated with BAC at either 0.10% or 0.08%.