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The HGN Evaluation: How it works and how it doesn’t (Part 2)

The Horizontal Gaze Nystagmus (HGN) Evaluation is the first of the three standardized field sobriety evaluations discussed in the first blog of this series.  It is generally regarded as the most accurate and effective evaluation so long as it is done by a qualified, well versed officer.  This blog will discuss the HGN Evaluation; how it works, and how defense attorneys can attack it.


The HGN Evaluation is the only “scientific” evaluation.  When performed correctly, what you see is what you get.  There are no external variables that can impact the evaluation.  If clues are present, with very few exceptions, impairment can be interpreted (even a blood alcohol concentration (BAC)) with extreme accuracy.


Nystagmus is an involuntary jerking of the eye.  What an officer is looking for is the eyes bouncing when tracking left to right at different points and angles.  It will pick up any Central Nervous System (CNS) depressant (alcohol and some prescription drugs).


The evaluation is divided up into three phases (which I have renamed for explanation purposes):


  1. (Medical) Clearance Phase
  2. Testing Phase
  3. Tolerance Phase


During the first phase, the officer determines whether the subject is qualified to participate in the evaluation.  During this phase, the officer swipes a stimulus (finger or pen) horizontally across the line of sight of a driver.  The subject is told to keep his head still and move only his eyes.  During this phase, the officer is looking for resting nystagmus (eyes moving erratically without movement side to side while tracking a stimulus), equal pupil size, and that both eyes move to follow the stimulus.  If the officer fails to see resting nystagmus, sees equal pupil size and equal eye tracking, the subject is clear to perform the evaluation.  If the officer does not, this may suggest a medical condition, head injury or other issue that would prevent the officer from moving forward.


Assuming that the officer medically qualifies the participant, he/she will move on to the validated portion of the evaluation.  There are three validated portions.


  1. Lack of Smooth Pursuit
  2. Distinct and Sustained Nystagums at Maximum Deviation
  3. Onset of Nystagmus Prior to 45 Degrees


Generally, there are two clues for each portion of the evaluation; one clue for each eye for a total of 6 possible clues.  Each portion of the evaluation has very stringent timing requirements for how quick the passes are made to the left and right; how long the stimulus can be held in a particular area; and how far the stimulus can be held from the subject.  The more clues you see going through the first, second, and third portions, the more impaired the subject is.  Assuming that the officer does everything properly, NHTSA has determined that the HGN evaluation is 77% accurate in distinguishing BAC’s above .10.  There are a select group of people called Drug Recognition Experts who go through a 6 month training regimen that can utilize this evaluation to fairly accurately tell what a person’s BAC is simply by observing where the nystagmus happens in the field of travel.  This is called Tharps Theorem, which is 50 – (angle of onset).   Therefore, if the DRE estimates the angle of onset at 35 degrees, 50-35=15. Therefore the subjects BAC should be near a .15. This is a very advanced procedure and is rarely allowed into evidence.


Finally, the last portion is not HGN, it is actually Vertical Gaze Nystagmus (VGN) where the officer moves the stimulus up and down instead of left and right.  This is not a validated portion of the evaluation; however, if the officer sees VGN, it suggests that the substance ingested is a high amount for that particular individual.  Obviously, that will vary from person to person which is why I call this the Tolerance Phase.


Although a very powerful tool in the officers “bag”, HGN can be attacked by an attorney effectively if the attorney knows what he/she is looking for.  As mentioned above, if the officer doesn’t follow the timing requirements or other standardized protocol designated by NHTSA, the officer’s own manual says that the results are “compromised.”  It’s possible to have the results of the evaluation thrown out or even use the officers lack of proficiency in that one evaluation to “compromise his/her entire investigation. Our office has been heavily involved in officer training on DUI Detection and Enforcement as prosecutors and we are also Standardized Field Sobriety Trained.  We can identify these issues and develop effective arguments to attack the results of the HGN evaluation.


If you have been charged with a DUI, call our office immediately for a consultation.


(770) 945-2320

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We are located in Suwanee Georgia, but we defend clients throughout Gwinnett County, Forsyth County, Hall County, Barrow County, Jackson County, and Dawson County.  Contact our Atlanta law firm as soon as possible to learn how we advocate for your best interests.

Carter Pilgrim
Attorneys at Law

3725 Lawrenceville-Suwanee Rd, Suite A-4
Suwanee, GA 30024

Phone: Call our office 770-945-2320

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