Challenging the Reliability of the Horizontal Gaze Nystagmus (HGN) Test in Court

If an officer pulled you over in Overland Park and asked you to follow a pen or finger with your eyes, you experienced the Horizontal Gaze Nystagmus test. Police use the HGN test more than almost any other roadside sobriety check, and prosecutors often treat it as reliable proof of impairment. But the reality is far more complicated, and knowing how courts scrutinize this test can make a real difference in a DUI case.

What Is the HGN Test and Why Do Officers Use It?

The HGN test measures involuntary eye jerking triggered by alcohol. Officers use it roadside because it requires no equipment and takes less than 2 minutes to complete.

Nystagmus is the involuntary jerking of the eye that happens when the eyes track a moving object. Alcohol suppresses the central nervous system, which can exaggerate this movement. An officer administering the HGN test looks for three specific clues in each eye:

• Lack of smooth pursuit as the eye follows the stimulus
• Distinct and sustained nystagmus at maximum deviation
• Onset of nystagmus at a 45-degree angle

Under the National Highway Traffic Safety Administration (NHTSA) standardized field sobriety test protocol, the presence of four or more clues out of six is considered an indicator of a blood alcohol concentration at or above 0.08 percent. Kansas statute K.S.A. 8-1012 establishes 0.08 percent BAC as the legal limit for most drivers, so this threshold matters.

The problem is that the HGN test is only as good as the conditions under which it is administered and the officer who administers it, and as to whether its scientific reliability has been properly established in court under Kansas law.

Is the HGN Test Scientifically Reliable?

HGN has scientific backing, but its accuracy in real-world roadside conditions is far weaker than lab results suggest, and courts have recognized this.

The NHTSA claims a properly administered HGN test can identify alcohol impairment with roughly 77 percent accuracy when used as part of the Standardized Field Sobriety Test battery. That figure alone should raise questions. A 23 percent error rate means that a meaningful number of sober people would fail this test under ideal conditions.

Roadside conditions are rarely ideal.

The Kansas Supreme Court held in City of Wichita v. Molitor, 301 Kan. 251 (2015), that HGN is based on scientific principles and cannot be considered by Kansas courts for any purpose unless and until the State first proves the test’s scientific reliability in a Kansas district court. Because that rigorous scientific foundation has generally not been met in practice, HGN results are almost never admitted at trial and cannot even be used to establish reasonable suspicion for a preliminary breath test.

What Can Cause a False Positive on the HGN Test?

Dozens of medical conditions, medications, and environmental factors can trigger nystagmus that has nothing to do with alcohol consumption.

This is one of the most powerful arguments in an HGN challenge. Nystagmus is not exclusive to alcohol impairment. Other causes include:

• Certain prescription and over-the-counter medications, including antihistamines and anticonvulsants
• Inner ear disorders and conditions like vertigo
• Head injuries or neurological conditions
• Caffeine, nicotine, and aspirin at high doses
• Fatigue and visual disturbances

If the administering officer never asked about medical history or current medications, that failure can become a key issue in court. NHTSA’s own training manual instructs officers to note any medical conditions that might affect the test, and skipping that step undermines the result.

How Defense Attorneys Challenge HGN Evidence in Court

Defense attorneys attack HGN evidence by challenging officer training, test administration, and the exclusion of alternative causes of nystagmus.

Challenging HGN evidence is a layered process. It starts with a careful review of the officer’s training and certification. NHTSA requires specific training for officers to administer standardized field sobriety tests. If the officer’s training was outdated, incomplete, or did not follow current NHTSA protocols, the results can be challenged on that basis alone.

The physical conditions of the stop matter too. Passing traffic creates flashing lights that can interfere with eye tracking. An uneven surface can affect a driver’s balance and posture, which indirectly affects how nystagmus presents. Even the way the officer held the stimulus, whether it was moved too quickly, held at the wrong distance, or not kept at the correct height, can invalidate the result.

Body camera and dashcam footage are often critical here. If the footage contradicts the officer’s written account of how the test was administered, that discrepancy carries weight before a judge or jury.

A DUI defense attorney who understands both the science and the procedural requirements can examine each of these variables and identify where the test broke down.

Does the HGN Test Hold Up as the Only Evidence?

Prosecutors rarely rely on HGN alone, but when other evidence is weak, a successfully challenged HGN result can significantly undermine the state’s case.

In most DUI prosecutions in Johnson County, the HGN result is one piece of a larger picture that might include a breathalyzer reading, officer observations, and other field sobriety tests. But there are cases where the chemical test result is disputed or unavailable, and the field sobriety evidence carries more weight.

When HGN is central to the prosecution’s theory, attacking its foundation becomes that much more important. Suppressing or discrediting HGN evidence does not automatically win a case, but it can reshape how a jury weighs everything else.

Talk to Barnds Law LLC About Your DUI Defense

At Barnds Law LLC, we do more than defend cases; we make sure our clients understand what is happening at every step and what to expect going forward. Our team of attorneys approaches each case collaboratively, bringing multiple perspectives to find the most effective strategy for your situation. We believe that an informed client is a stronger client, and that philosophy drives how we work.

If the HGN test was part of your traffic stop in Overland Park or anywhere in the Kansas City metro area, contact us or call us at 913-514-0909 to discuss your case.