Field Sobriety Tests Predict Blood Alcohol Content

​​Q: Why do police officers make suspected drunk drivers do roadside balance acts?
A: The balance acts are actually “Standardized Field Sobriety Tests” (SFST) and include: 
1) Horizontal Gaze Nystagmus (HGN);
2) Walk and Turn (WAT); and
3) One Leg Stand.

SFSTs were developed in the late 1970s and early 1980s as a way for law enforcement to establish a standardized method to help them determine whether an individual is too impaired to drive. Rather than serving as a test for impairment, the tests actually predict the probability that an individual’s Blood Alcohol Content (BAC) is higher than .08. 

Q: What is Horizontal Gaze Nystagmus (HGN)?
A: Horizontal Gaze Nystagmus (HGN) is a jerking of the eye as the eye moves horizontally while following a moving stimulus. You may not feel it and may not know that it is occurring. There are three parts to this test. The officer holds a stimulus (usually a pen) 12 to 15 inches from the driver’s nose and moves it horizontally across the driver’s field of vision. The officer first checks for nystagmus (jerking) as the driver's eyes shift to the left and right, and then checks for nystagmus while holding the stimulus to the far right and then to the far left of the driver’s field of vision. Finally, the officer looks for the onset of nystagmus before the stimulus reaches a 45-degree angle from the starting point (the driver’s nose). If the officer observes a total of at least four of the six possible clues (three clues in each eye), there is a 77 percent probability that the individual would test at a .08 BAC or higher. 

Q: What are the Walk and Turn and the One Leg Stand tests?
A: The Walk and Turn and One Leg Stand Tests are balance and coordination tests that also determine an individual’s ability to follow instructions (accompanied with demonstrations). However, the instructions are long and may be confusing, and the officer may have trouble giving proper instructions. The Walk and Turn has a 68 percent probability of predicting that an individual has a BAC of .08 or higher and the One Leg Stand is accurate about 65 percent of the time.

Q: Are these field sobriety tests admissible in court?
A: Yes. Although many courts consider the Walk and Turn and One Leg Stand to be non-scientific tests, findings from these tests are generally admissible as evidence in court. Further, courts often hold that the Horizontal Gaze Nystagmus test is scientifically reliable if administered in strict compliance with National Highway Traffic Safety Administration (NHTSA) standards. If, however, it can be shown that any one of these tests was not administered correctly, the findings may be inadmissible in court. 


This "Law You Can Use" consumer legal information column was provided by the Ohio State Bar Association. It was originally ​prepared by attorney Jon J. Saia, a partner in the Columbus law firm, Saia & Piatt, Inc., and updated by Jessica G. D'Varga of the same firm.  

Articles appearing in this column are intended to provide broad, general information about the law. This article is not intended to be legal advice. Before applying this information to a specific legal problem, readers are urged to seek advice from a licensed attorney.



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