The eye test police use during a DUI stop is called the Horizontal Gaze Nystagmus test, or HGN. It measures the involuntary jerking of your eyes as they follow a moving object from side to side. Alcohol and certain other drugs make that jerking more pronounced and cause it to start earlier in the eye’s range of motion. Officers look for three specific signs in each eye, for a possible total of six. If they see four or more, NHTSA research puts the chance that your blood alcohol concentration is at or above 0.08 at roughly 88 percent.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual That makes it the most accurate single roadside test officers have, but it is not conclusive on its own.
What Nystagmus Is
Nystagmus is an involuntary bouncing of the eyes. You cannot feel it happening and you cannot stop it. Everyone has a small amount of it when they look as far to one side as their eyes can go. Alcohol and other central nervous system depressants interfere with the brain’s control of the eye muscles, so the jerking becomes more obvious and starts sooner as the eyes move toward the side.2National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Manual
The higher the BAC, the earlier and stronger the nystagmus. That predictable pattern is what the test relies on. Unlike balance tests, you cannot concentrate or practice your way through it. The eyes do what they do.
How the Test Is Given
The officer will ask you to stand with your feet together, hands at your sides, and keep your head still. You follow a small object with your eyes only. The stimulus is usually a pen, penlight, or fingertip, held about 12 to 15 inches from your nose and slightly above eye level.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
Before scoring begins, the officer runs two quick checks. They look at your pupils to see whether they are equal in size and whether your eyes jerk at rest. They also move the stimulus rapidly side to side to see whether both eyes track together. Unequal pupils or eyes that will not move in sync can point to a medical problem, and the officer may stop the test there.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
Then the officer makes three passes for each eye, one pass per clue, always starting with your left eye. Each clue is checked at least twice.
The Three Clues the Officer Scores
Each eye is scored on the same three signs, which is where the maximum of six comes from.
Lack of smooth pursuit. The officer moves the stimulus from the center of your face out to the side at a steady pace, taking about two seconds to reach the end. A sober eye glides along with it. An impaired eye visibly bounces or stutters as it tracks. NHTSA training compares the look to a windshield wiper on a dry windshield.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
Distinct and sustained nystagmus at maximum deviation. The officer moves the stimulus as far to the side as your eye can follow and holds it there for at least four seconds. A small flutter at that extreme is normal. Obvious, continuous jerking that does not settle counts as a clue.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
Onset of nystagmus before 45 degrees. As the stimulus moves outward, the officer watches for the first point at which the eye starts to jerk. If that point comes before the eye reaches a 45-degree angle from center, it scores. The earlier the onset, the higher the likely BAC. According to NHTSA, onset before 45 degrees on its own suggests a BAC above 0.08.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
What the Score Actually Proves
The threshold is four. Four or more clues across both eyes tell the officer there is a high likelihood your BAC is 0.08 or higher. NHTSA’s field validation research put the classification accuracy at about 88 percent.2National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Manual
That is the highest accuracy of any individual roadside test, better than either the walk-and-turn or the one-leg stand. It is not perfect. Roughly one person in eight who hits four clues may actually be under the legal limit. For that reason, HGN results are rarely used alone to support a DUI arrest or conviction. Officers combine them with the other field sobriety tests, with chemical testing after arrest, and with their observations of your driving, speech, and behavior. When all three standardized field tests are used together, NHTSA reports officers made correct arrest decisions about 91 percent of the time at the 0.08 threshold.2National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Manual
What Else Can Cause Nystagmus
The 88 percent figure comes from controlled studies with trained officers following the protocol exactly. On a real roadside, several things unrelated to drinking can produce the same eye movements.
Medical Conditions
A long list of medical problems causes nystagmus on its own. Inner ear disorders such as vestibular neuritis, labyrinthitis, and Ménière’s disease are common sources. Neurological conditions including multiple sclerosis, brainstem stroke, and cerebellar degeneration can produce it. So can a past head injury.3National Library of Medicine. Nystagmus Types – StatPearls
Prescription Medications
Several widely prescribed drugs are documented nystagmus triggers. Anti-seizure medications such as carbamazepine, lamotrigine, and phenytoin are among the most significant.4PMC (PubMed Central). Antiseizure Medication-Induced Nystagmus During Eye Closure Identified by Electroencephalography Lithium, sedatives, and certain antibiotics in the aminoglycoside family can do the same.3National Library of Medicine. Nystagmus Types – StatPearls A completely sober person on one of these medications can display multiple HGN clues.
The Setting and the Officer
Flashing lights from the patrol car, headlights from passing traffic, and poor roadside lighting can all affect eye behavior during the test. Officer technique matters as well. Holding the stimulus too close or too far, moving it too quickly, or failing to hold it at maximum deviation for the full four seconds all undermine the result. The accuracy numbers assume the protocol is followed closely.
Can You Refuse the Eye Test?
In most states, field sobriety tests, including HGN, are voluntary. You generally face no separate legal penalty for declining to perform them at the roadside. Refusing will not necessarily stop an arrest. The officer can still arrest you based on other observations such as the smell of alcohol, slurred speech, or how you were driving.
A chemical test after arrest is different. Under implied consent laws in every state, driving means you have already agreed to submit to a breath or blood test if you are lawfully arrested for impaired driving. Refusing that post-arrest chemical test typically triggers automatic license suspension and can bring enhanced penalties if you are later convicted. In Birchfield v. North Dakota, the Supreme Court ruled that states can require a warrantless breath test after a DUI arrest but cannot criminally punish someone for refusing a blood draw without a warrant.5Justia US Supreme Court. Birchfield v North Dakota The roadside eye test and the station-house breathalyzer are not the same decision, and the consequences of refusing them are very different.
How HGN Holds Up in Court
Whether an HGN result can be used against you at trial depends on your state. Most states treat it as scientific evidence that must meet the local standard for reliability before a jury hears it. Federal courts and many state courts apply the Daubert standard, which asks whether the technique has been tested, peer-reviewed, and generally accepted in the scientific community.6Legal Information Institute. Daubert Standard Other states still apply the older Frye standard.
Most states that allow HGN evidence draw a clear line. The officer can testify that your eyes showed clues consistent with impairment. The officer cannot tell the jury that your BAC was a specific number based on where nystagmus began. A few states, such as Mississippi, have excluded HGN evidence entirely.7National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus – The Science and the Law
Defense challenges most often focus on procedure. If the officer moved the stimulus too fast, did not hold at maximum deviation for the full four seconds, or ignored a documented medical condition, the reliability of the result is open to attack. The accuracy claims depend on strict adherence to the protocol, so any deviation gives the defense room to argue the clues should not count.