All You Need to Know About the Horizontal Gaze Nystagmus (HGN) Test 

Legally reviewed by:
Rubin, Glickman, Steinberg & Gifford P.C.
July 15, 2021

All You Need to Know About the Horizontal Gaze Nystagmus (HGN) Test In Pennsylvania, drivers pulled over by law enforcement because they are suspected to be under the influence of drugs or alcohol may be asked to perform sobriety tests on the side of the road, known as standardized field sobriety tests (SFSTs). The purpose of these tests is to obtain probable cause to make an arrest for driving under the influence (DUI), after which police can take drivers to the station to conduct a breath or blood test. In these instances, police are permitted to request that drivers perform up to three tests: the horizontal gaze nystagmus (HGN) test, the walk-and-turn, and the one-leg stand—in that order. If you find yourself in this situation in Pennsylvania, the content below explains everything you should know about the HGN test and how police use it to establish probable cause for an arrest. 

How Do Horizontal Gaze Nystagmus Tests Work?

Police officers across the United States have been using SFSTs since 1977 after the National Highway Traffic Safety Administration (NHTSA) developed them as a way for police officers to make objective observations to deduce whether a driver is intoxicated.

Nystagmus is the technical term for an involuntary movement in the eye’s pupil and iris. Researchers have discovered that the eye has horizontal nystagmus when individuals have a significantly high blood alcohol content (BAC). When police conduct an HGN test at a DUI stop, they’re analyzing your eye movements and looking for a twitch or other movement that may suggest that you’ve had too much to drink before getting behind the wheel. The HGN test is just one of the multiple ways that Pennsylvania law enforcement officers might evaluate a driver’s intoxication level.

What Clues Do Police Look for During the HGN Test to Suggest Drunk Driving?

While conducting an HGN test, police are looking to see if the driver’s eyes move ordinarily. In other words, they’re looking to see if the driver’s pupils jerk back and forth when their gaze moves towards the corner of their eyes. As a person becomes more intoxicated, their pupils involuntarily jerk back and forth when their gaze shifts towards the corners of their eyes. After drinking a significant amount of alcohol, these movements become increasingly prominent.

Note that the twitching movement is unintentional and can’t be controlled by an intoxicated person. When police administer an HGN test, they’re looking for the following clues to indicate drunk driving:

  • The extent to which the eyes jerk 
  • How soon the eyes start to jerk back and forth as the gaze moves towards the corners of the eyes
  • Whether the driver moves only their eyes or their whole head while following the officer’s instructions
  • The driver’s ability to follow the police officer’s directions

If you’re in this situation and the officer believes you’re evading the test or refusing to follow instructions, they may consider it to be an indication that you’re trying to fool the test or hiding something.

How Do Police Officers Administer the HGN Test in Pennsylvania?

Police officers in Pennsylvania administer the HGN test initially by instructing an allegedly intoxicated driver to watch their pen, finger, or another object in front of their eyes as they move it from left to right. Whatever object they chose, they are supposed to hold it about one foot away from the driver’s eye level.

The officer then instructs the driver to focus on the object as they shift to the right and left. At this point, police are searching for twitching movements when the pupils of the eyes are in the corners. After moving the object from side to side, they may also move it up and down. If the driver’s eyes twitch while looking up and down, they may be suspected of having a high level of alcohol in their system.

How Reliable Are HGN Tests in a DUI Case?

HGN tests are not a surefire method to determine a person’s level of intoxication. In fact, it does not determine a driver’s BAC at all. Instead, it’s just one of the many tools law enforcement uses to gauge a driver’s sobriety. If a driver fails the HGN test by showing signs of intoxication, that will provide probable cause for the officer to conduct official DUI tests at the police station that may include a: 

  • Breath test
  • Blood alcohol test, or 
  • Urine test 

To produce reliable results showing a person’s intoxication level, the officer must administer it perfectly. Otherwise, it may be a false indication of the driver’s level of intoxication. Further, if the officer holds their pen too far or too close to an allegedly drunk driver, or if they move too fast, the results may be inaccurate.  

Law enforcement officers must be suitably trained to read an HGN test correctly. Otherwise, the results may not hold up in court as probable cause to investigate a potential DUI further when scrutinized by an experienced DUI defense attorney. Keep in mind that the HGN test is just one of three SFSTs that must be combined with a thorough inquiry to provide sufficient probable cause to test a driver’s insobriety further.

Contact an Experienced Pennsylvania DUI Defense Lawyers in Montgomery County

If you’ve been arrested for DUI, do not delay retaining a lawyer as soon as possible after your arrest. At Rubin, Glickman, Steinberg & Gifford, P.C., our Pennsylvania DUI defense lawyers realize how challenging this experience can be for you, which is why we’re devoted to advocating for the rights and freedoms of individuals through Pennsylvania’s judicial process. Whether it’s your first DUI arrest or your third, we are here to help you navigate every step of this process.  

For over 65 years, Rubin, Glickman, Steinberg & Gifford, P.C.’s criminal defense attorneys have represented clients in Montgomery County, Bucks County, and Southeastern Pennsylvania. With a positive track record and insightful knowledge of Pennsylvania’s intricate DUI law, our attorneys have earned various peer recognition awards and prestigious honors, including an AV-Rating by Martindale-Hubbell and recognition by Best Lawyers in America. To consult one of our skilled DUI defense lawyers, schedule your free case review by completing a contact form or calling 215-822-7575.

Legally reviewed by:
Rubin, Glickman, Steinberg & Gifford P.C.
Pennsylvania Attorney's
July 15, 2021
Established in 1952 by Irwin S. Rubin, Rubin, Glickman, Steinberg & Gifford P.C. boasts over 65 years of experience serving clients throughout Pennsylvania. Renowned for its commitment to ethical representation, the firm has garnered prestigious accolades, including being named the "Best Law Firm" for its outstanding legal defense work by U.S. News & World Report. Their team of seasoned attorneys, recognized as Pennsylvania Super Lawyers and Rising Stars, brings unparalleled expertise to a wide range of legal matters, ensuring exceptional representation for individuals, families, businesses, and organizations.