Field Sobriety Tests: The Horizontal Gaze Nystagmus (HGN) Test

Field sobriety tests (FST) are used during an OWI traffic stop so that the police officer can collect enough evidence to meet his or her requirement of obtaining enough probable cause to legally take a sample of your blood, breath, or urine. Although there are many different types of field sobriety tests, the courts and scientific research have found that only three have enough support to prove that if two or more clues are seen then there is a high likelihood that the person has a blood alcohol content that is above the legal limit. The first test we will go over is the Horizontal Gaze Nystagmus (HGN) test. With the HGN test, an officer uses a pen, light, or their finger to watch the hooking or movement of your eyes. When this test is administered, the police officer normally begins with your left eye. Each eye is examined for three specific clues: 1) as the eye moves from side to side, does it move smoothly or does it jerk noticeably. 2) when the eye moves as far to the side as possible and is kept in that position for several seconds, does it jerk distinctly, and 3) as the eye moves toward the side does it start to jerk prior to a forty-five (45) degree angle? Alcohol impairment can cause your eyes to do any of those three clues. The maximum number of clues that can happen in one of your eyes is three, so that means the total number of clues that can occur are six.  Research has shown that if four or more clues occur, it is likely that your BAC is above .10. However, there are 96 different types of nystagmus, which can occur for a variety of reasons from pre-existing issues from brain or eye trauma to lack of sleep. When done properly, the HGN test has a 70% chance of predicting alcohol impairment according to the National Highway Traffic Safety Administration. However, officers are often not re-tested on their initial training and execute the standard procedures poorly. On the usual dash camera video, it is difficult to determine the actual movement of the driver’s eyes, making the challenging of the results more difficult. However, often when officers are called to the witness stand they will struggle explaining standard testing procedure.  If the officer does not state the procedures correctly, then you could argue that the test should not be valid nor used against you in your case. Contact us today to set up a free consultation to discuss how the officer in your case conducted the HGN test in your case.