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Horizontal Gaze Nystagmus Test (HGN), Why HGN Cannot Be Deemed Reliable

The National Highway Transportation Safety Administration, NHTSA, has come up with three “standardized field sobriety tests” for law enforcement to use to aid in drunk driving investigations. One of the three tests is the horizontal gaze nystagmus, or HGN, test. (The HGN test was previously discussed in “Horizontal Gaze Nystagmus Test (HGN), How It Is Used In DUI Investigations.”)

Law enforcement is supposed to administer the HGN test using the guidelines that NHTSA has laid out to ensure the results are valid and reliable. Cops look for six clues, three in each eye. If four clues are found, the officer can determine that the subject’s blood-alcohol content is at or above a .10%.

There are many problems with the HGN test and how it is used by law enforcement to determine if someone is under the influence when conducting a DUI investigation. First off, many people have a natural nystagmus. Prior to the encounter in question between the cop and the suspect, the degree of the natural jerk or bounce in the eye is not known. Therefore, the officer cannot accurately correlate this to impairment.

Another issue is that many drunk driving investigations are done at night, on the side of the road with minimal lighting, with cars passing, and with the cop’s flashing lights on. This is an issue because lighting can affect the results of the test.

However, the biggest and most concerning issue with the HGN test comes from the 1983 NHTSA study that was conducted to prove that HGN and blood-alcohol content are related. NHTSA actually funded this study with the hopes of proving their hypothesis that the HGN test was valid and reliable. This presents a conflict of interest and is a biased study. According to ordinary scientific principles, an independent study by an unbiased group should have funded and conducted the research.

Additionally as alarming, the study found that the HGN test was 77% accurate in detecting whether a person’s blood-alcohol content, or BAC, was .10 or higher. This means that the findings actually proved that almost 1 in 4 people who law enforcement deemed to have a BAC of .10% or higher was in fact under the .10 standard!

So, assuming the officer conducts the test correctly using the NHTSA’s guidelines, the subject does not have a natural nystagmus, and the test is performed with proper lighting, according to the study, almost 1 out of every 4 people tested would be found to be under a .10% BAC. (Note: There has not been a study done using the current legal limit of .08%.) With such issues, this evidence should never come into a criminal courtroom to prove a person was under the influence for purposes of driving a motor vehicle…but it does.

The above blog article is by no means all-inclusive and is not legal advice. Laws may change and may not apply to your case. For the latest information or to get legal advice, speak to a DUI attorney in your area.


The Law Offices of Susan L. Hartman understands the nuances of DUI defense and fights hard for clients that are facing drunk driving charges. We offer a free telephone consultation. If you have a pending drunk driving charge, contact our office at 619-260-1122, or use the “Contact Us” form on this page.