Quick answer: The HGN test is the “follow my pen” eye test — an officer moves a stimulus and watches for involuntary jerking that alcohol can exaggerate. In Maryland, how it gets used is the key: under State v. Blackwell, HGN is expert testimony, and a judge can only let a jury hear it after the State lays a proper foundation. It is evidence of impairment or the presence of alcohol — it never produces a BAC number.
I’m David Waranch, and I’ve defended DUI cases that turned on how the officer ran this one test.
What the HGN Test Actually Is
Horizontal gaze nystagmus is an involuntary jerking of the eye as it moves toward the side. Everyone’s eyes do it a little at the far edges of vision; alcohol can make the jerking start sooner and become more pronounced. In the field, the officer holds a pen or small light about a foot from your face and asks you to follow it with your eyes only, head still. As it moves, he watches each eye for three specific clues — six in all. Four or more is the standard cutoff, which NHTSA training treats as consistent with a BAC of 0.08 or higher — a probability the training claims, not a measurement the test performs.
HGN is one of the three Standardized Field Sobriety Tests developed for the National Highway Traffic Safety Administration, alongside the walk-and-turn and the one-leg-stand. This page covers the eye test alone; for how the full battery fits together, see my overview of the three field sobriety tests.
| Clue | What the officer is looking for |
|---|---|
| Lack of smooth pursuit | As the eye tracks the stimulus, it jerks or bounces instead of moving smoothly, like a windshield wiper across a dry window. |
| Distinct, sustained nystagmus at maximum deviation | With the eye turned as far to the side as it will go and held there, it jerks distinctly rather than sitting still. |
| Onset of nystagmus before 45 degrees | The jerking begins while the eye is still short of a 45-degree angle from center — not out at the far corner. |
The common shorthand of “watch the pen at the corner and center of your vision” gets the last two clues wrong. Clue two is scored at maximum deviation and clue three turns on the angle of onset — distinctions that matter, because an officer who cannot describe them correctly usually did not administer the test correctly either.
What HGN Can and Cannot Establish
The most important thing about this test is the limit on what it proves. HGN can be evidence that a driver is impaired or has alcohol in the system, but it does not give a number. No officer can look at your eyes and tell you your BAC, and Maryland courts do not pretend he can. What the numbers mean is a chemical-test question — see how blood alcohol concentration works in Maryland.
| HGN can be offered as evidence of… | HGN cannot establish… |
|---|---|
| Impairment, or the presence of alcohol | A specific blood-alcohol concentration |
| One indicator among several the officer observed | Guilt on its own, without the rest of the case |
| Reasonable grounds to make a DUI arrest | That the jerking was caused by alcohol rather than something else |
The Maryland Rule: State v. Blackwell
In State v. Blackwell, 408 Md. 677, 971 A.2d 296 (2009), Maryland’s highest court settled how HGN gets into evidence. The holding: HGN-test testimony is expert testimony, not something an officer can simply narrate the way he describes watching you sway. Because it is expert testimony, it is admissible only under Maryland Rule 5-702, which requires the trial judge to make a preliminary determination before the jury ever hears the result.
That preliminary determination — the “foundation” — has two parts: the State must show the officer was trained and qualified to administer HGN to the recognized standard, and that on this occasion the test was administered properly. If it cannot lay both, the judge should keep the HGN evidence out. This is real gatekeeping, and often the first place a defense goes to work — the foundation is skipped or rushed more often than people expect.
One caveat: at an MVA hearing rather than criminal court, the bar is only reasonable grounds to believe you were impaired — lower than the criminal foundation. That track runs separately and in parallel; the per se hearing works on its own rules.
“Failing the eye test proves I was drunk.” It proves nothing by itself. HGN is expert evidence of impairment that a judge must first find admissible — not a breath reading, and no BAC. Before a jury can even weigh it, the State must show the officer was properly trained and ran the test correctly. Plenty of HGN results never clear that bar, and even those that do are one clue among many, not a verdict.
How the HGN Test Gets Challenged
Because Blackwell makes HGN turn on foundation and proper administration, the defense angles follow from those two requirements:
- No foundation was laid. If the State never establishes the officer’s training or proper administration, the evidence should not reach the jury.
- The officer was not certified to standard. HGN scoring depends on NHTSA-standard training; an officer without current certification is not qualified to give the expert testimony.
- The test was administered incorrectly. Moving the stimulus too fast, holding it too close, or not holding at maximum deviation long enough all undercut the result.
- Something other than alcohol caused the jerking. Inner-ear problems, certain medications, neurological conditions, fatigue, and even roadside headlights and flashing emergency lights can all produce or mimic nystagmus.
An experienced Maryland DUI attorney reads the officer’s report and the in-car video against the protocol line by line. When the administration does not match the training, the result becomes far less persuasive — and sometimes inadmissible.
Do You Have to Take the Eye Test?
The field sobriety tests, HGN included, are done at the roadside before an arrest, and in practice drivers are free to decline them — there is no statutory penalty for saying no the way there is for the post-arrest chemical test. The breath or blood test after arrest falls under Maryland’s implied-consent law, Md. Code, Transp. § 16-205.1, and refusing that one carries its own license suspension. Declining a roadside pen test is not the same as refusing the breathalyzer after arrest. If you are already charged, the useful question is what a lawyer can do with the case you have — even when the numbers look bad.
If an HGN result is part of your case, call me at 301-563-9575 and we will go through exactly what the officer did and whether it holds up.
Frequently Asked Questions
No. The HGN test can be evidence that a driver is impaired or has alcohol in the system, but it does not produce a blood-alcohol concentration. Only a chemical breath or blood test yields an actual number. Any suggestion that the eye test measured your BAC misstates what the test can do.
It can be, but not automatically. Under State v. Blackwell, HGN is expert testimony admissible only under Maryland Rule 5-702, after the trial judge makes a preliminary finding that the officer was trained and qualified and that the test was administered properly. If the State cannot lay that foundation, the evidence should be kept out.
The roadside field sobriety tests, including HGN, are done before arrest and are treated as voluntary in practice, with no statutory penalty for declining them. That is different from the post-arrest chemical breath or blood test under the implied-consent law, which does carry a license suspension if you refuse it.
Law Offices of David R. Waranch — 301-563-9575 · toll-free 1-877-566-2408. Hablamos Español.
Last updated: August 2026. Authorities: State v. Blackwell, 408 Md. 677, 971 A.2d 296 (2009); Md. Rule 5-702; Md. Code, Transp. §§ 16-205.1, 21-902. This page is general information about Maryland law, not legal advice, and reading it does not create an attorney-client relationship.