An accurate estimate can be made from HGN testing?

• A. True
• B. False
• C. Usually true
• D. Sometimes depending on lighting

Answer: (B)

Horizontal Gaze Nystagmus is a qualitative indicator used in the Standardized Field Sobriety Test to show that impairment from alcohol (and some drugs) is likely. It does not provide a precise measurement of how impaired someone is or their exact blood alcohol concentration. There’s substantial individual variability in how people’s eyes respond to alcohol, and other factors—such as fatigue, medication, medical conditions, or even eye injuries—can affect the observed jerking. Because of this variability, you can’t rely on HGN alone to give an accurate estimate of impairment. It’s best used in conjunction with the other SFST clues and the broader assessment to decide whether to pursue further testing (like a breath or blood test) and potential arrest.