If you are stopped by law enforcement and they suspect that you might driving while intoxicated, you may be asked to complete a series of field sobriety tests, such as reciting the alphabet, balancing on one leg, or walking in a straight line. While these activities test your mental state and physical coordination, it can be more difficult to understand what the horizontal gaze nystagmus (HGN) test accomplishes.

How The Horizontal Gaze Nystagmus Test Works

During the horizontal gaze nystagmus test, the police officer will ask you if you have any medical issues related to your eyes, and take you away from glaring traffic and any flashing police cruiser lights. In a well-lit area, or with the aid of a flashlight, they will have you look directly ahead, and then have your gaze follow an object, like their finger.

As your eyes follow the object to the side of your head, the officer will observe any nystagmus in your eye – a bouncing or jerking that often occurs in intoxicated people when the eye’s gaze moves to a 45 degree angle. Specifically, the officer will look for:

  • Any abnormalities in the eyes that may affect the accuracy of the test.
  • Nystagmus when the eye is at rest.
  • Smooth pursuit – if the eyes follow the object smoothly.

People who are intoxicated often display nystagmus when intoxicated.

Is The Horizontal Gaze Nystagmus Test Accurate?

According to tests conducted by the National Highway Traffic Safety Administration (NHTSA), the HGN test is only about 77 accurate – and that’s if it’s administered completely correctly. Because of its lack of accuracy, it’s most often used in combination of other field sobriety tests, in hopes that all the tests taken together will help officers determine the state of the driver.

It’s important to understand that a blood test is the most accurate way to determine blood alcohol content (BAC) – and that field sobriety tests have several drawbacks. The HGN test specifically can be inaccurate if the person has an eye condition or medical issue, if the lighting is poor, or if the officer does not correctly observe the driver’s eyes. Even if everything is executed correctly, it can have false results.

Hiring A St. Louis DWI Attorney For Your Drunk Driving Case

If you failed the horizontal gaze nystagmus field sobriety test, it does not necessarily mean that you were driving while intoxicated – it’s just a single piece of evidence in your case.  To learn more, get our FREE Missouri DWI Survival Guide.

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