The Horizontal Gaze Nystagmus (HGN) Test, when performed properly on a medically qualified individual, has a claimed 77% reliability rating. During the HGN evaluation the officer is looking for nystagmus of the eye. Nystagmus is simply the involuntary jerking of the eye. This involuntary jerking of the eye is often compared to the movement of windshield wipers on a dry windshield, or a marble on sandpaper.
To properly administer this test the officer must:
- Hold a stimulus (usually the officer's finger, or pen) 12 to fifteen inches from the subject's face)
- Hold the stimulus slightly above the subject's eye level
- Tell the subject to follow the stimulus with only their eyes making sure they hold their head completely still
- Move the stimulus smoothly in a straight line
- Initially, the officer must check each eye for equal pupil size, and equal tracking. This is done to see if the subject has suffered a concussion or has a glass eye
- The officer then checks each eye for 3 possibly clues
- The officer will check each eye twice for each set of clues.
During this evaluation, the officer is looking for 6 clues of impairment (2 clues per eye):
- Lack of smooth pursuit
- Distinct and sustained nystagmus at maximum deviation
- Onset of nystgamus prior to 45 degrees.
Unless a subject has a glass eye or a serious medical issue, an officer should never have an odd number of clues on the Horizontal Gaze Nystagmus (HGN) Test. The officer should always have 2, 4, or 6 clues. Also, each set of clues is progressive. Therefore, an officer should never notice distinct and sustained nystagmus at maximum deviation without noticing a lack of smooth pursuit, or onset of nystagmus prior to 45 degrees without observing the other four clues.
In summary, the National Highway Safety Transportation Authority (NHTSA) would have us believe that cumulatively, these 3 tests, if done correctly, have an 83% accuracy rate of indicating whether a subject has a BAC of .10% or more.
Knowledgeable DUI attorneys know that 98% of officers conduct these tests incorrectly, or on an unqualified or unapproved individual, or score them incorrectly. In some situations, officers make all 3 of these mistakes. When done incorrectly, these field sobriety tests have ZERO percent reliability