The HGN test, or Horizontal Gaze Nystagmus test, is a standardized eye exam that police officers use during traffic stops to check for signs of alcohol or drug impairment. It’s the first of three Standardized Field Sobriety Tests (SFSTs) developed by the National Highway Traffic Safety Administration, and it works by looking for involuntary jerking of your eyes as they track a moving object. Of the three standard roadside sobriety tests, HGN is considered the most reliable, with a 77 percent accuracy rate for identifying drivers at or above a 0.08 blood alcohol concentration.
Why Alcohol Makes Your Eyes Jerk
Your eyes normally move in smooth, controlled motions thanks to a reflex that coordinates eye movement with signals from your inner ear and brain. Alcohol disrupts this system at multiple levels of the nervous system, from the inner ear all the way up to higher brain functions. When you’re impaired, your eyes can no longer track a moving object in a fluid motion. Instead, they jump and stutter, producing an involuntary jerking called nystagmus. This jerking isn’t something you can feel or consciously control, which is what makes the test useful to law enforcement. Unlike walking a straight line or standing on one leg, you can’t will your eyes to behave normally when your nervous system is impaired.
How the Test Works
The officer begins by asking you to remove your glasses and stand with your feet together and hands at your sides. They’ll hold a small stimulus, usually a pen or penlight, about 12 to 15 inches from your nose, slightly above eye level. You’re told to follow the stimulus with your eyes only, keeping your head still.
Before the actual test begins, the officer checks a few baseline things: whether your pupils are equal in size, whether your eyes show any jerking while looking straight ahead (called resting nystagmus), and whether both eyes track together when the stimulus moves from side to side.
The test itself has three distinct phases, and the officer checks each eye separately for each one. Every phase is performed at least twice per eye.
The Three Clues Officers Look For
Each eye is evaluated for three specific indicators, giving a maximum of six total clues across both eyes.
Lack of Smooth Pursuit
The officer moves the stimulus smoothly from the center of your face out toward one side. A sober person’s eyes will follow the object in a fluid, continuous motion. An impaired person’s eyes will bounce or stutter as they track it, similar to how windshield wipers skip across a dry windshield. The stimulus moves at a pace of roughly two seconds from center to the side. If your eye jerks instead of gliding, that counts as one clue.
Distinct Nystagmus at Maximum Deviation
Next, the officer moves the stimulus to the far edge of your gaze and holds it there for at least four seconds. At this extreme position, they’re watching to see if your eye continues to jerk in a distinct, sustained way. Some minor twitching at the very edge of your vision can be normal, so the jerking needs to be clearly visible and persistent to count as a clue.
Onset of Nystagmus Before 45 Degrees
This is the most telling indicator. The officer slowly moves the stimulus from center toward the side, this time at a slower pace of about four seconds to reach a 45-degree angle. They’re watching for the exact point where the jerking first appears. In a sober person, nystagmus typically doesn’t begin until the eyes have moved well past 45 degrees. If the jerking starts before reaching that angle, it’s a strong sign of a blood alcohol concentration above 0.08.
What the Results Mean
With three clues possible per eye, the maximum score is six. The NHTSA threshold for indicating impairment is four or more clues out of six. Reaching that threshold doesn’t automatically prove you’re over the legal limit, but it gives the officer evidence to support a decision about probable cause.
The HGN test outperforms the other two standard field sobriety tests. It’s accurate about 77 percent of the time when used to identify a BAC of 0.08 or higher. By comparison, the walk-and-turn test is accurate about 68 percent of the time, and the one-leg stand comes in at 65 percent. When all three tests are used together, the combined accuracy improves, which is why officers are trained to administer the full battery rather than relying on any single test.
What Can Cause a False Positive
Alcohol isn’t the only thing that causes nystagmus, and this is one of the test’s key limitations. Several medical conditions and other factors can produce eye jerking that mimics impairment. These include neurological conditions and brain diseases, certain prescription medications (particularly anti-seizure drugs, sedatives, and some antidepressants), inner ear disorders, fatigue, anxiety, and even the specific lighting or background conditions at the scene. Age also plays a role, as the smooth pursuit system naturally becomes less precise as you get older.
If you have a medical condition that affects your eyes or take medications that could influence eye movement, this information can be relevant if your HGN results are ever challenged in court.
How HGN Holds Up in Court
The legal treatment of HGN results varies significantly by state. Courts generally handle HGN evidence in one of two ways. Some jurisdictions treat it as a simple physical observation, no different from an officer noting that a driver had bloodshot eyes or slurred speech. Under this approach, the officer just needs to show they were properly trained and administered the test correctly.
The majority of state courts, however, classify HGN as scientific evidence. This distinction matters because it triggers a higher standard for admissibility. The court may require the prosecution to demonstrate that the underlying science linking alcohol consumption to nystagmus is valid and generally accepted. In these jurisdictions, the officer can describe what they observed during the test but may not be allowed to testify that those observations indicate a specific level of impairment unless they’ve been qualified as an expert witness.
This split creates a meaningful difference in how much weight HGN results carry at trial. In states where it’s treated as a physical observation, the officer can offer their opinion on whether the driver appeared impaired. In states where it’s classified as scientific evidence, the officer’s testimony is more limited, and the prosecution may need additional expert testimony to connect the test results to actual impairment.
Common Mistakes in Administration
The test’s reliability depends heavily on proper technique. If the officer holds the stimulus closer than 12 inches from your face, they’re more likely to see jerking even in sober eyes. If they hold it farther than 15 inches, they may have trouble seeing the subtle eye movements at all. Moving the stimulus too quickly can make it impossible to identify the onset point of nystagmus, while moving it too slowly can produce fatigue-related jerking that has nothing to do with alcohol.
Each phase has specific timing requirements. The smooth pursuit check should take about two seconds from center to side. The onset check should take about four seconds to reach 45 degrees. Officers who rush through the test or skip the baseline checks for equal tracking and resting nystagmus are deviating from the standardized protocol, which can undermine the results if the case goes to court.