The part that trips people up most is that this is not a breath test and it does not give a blood-alcohol number. It is an eye test used by police during a roadside field sobriety test. An officer asks a person to follow a moving object, usually a pen or fingertip, and watches for involuntary jerking of the eyes as they move side to side. That jerking is called nystagmus. "Horizontal gaze" just means the officer is checking it while the eyes track left and right.

Practically, the test matters because officers use it to build probable cause for a DUI arrest. In Washington, it can come into court if the officer was trained and did the test correctly. Under State v. Baity (2000), HGN evidence may support impairment, but it does not prove a specific BAC. The actual per se DUI limit is set by RCW 46.61.502 (2024): 0.08 for most adult drivers.

What to do if HGN is part of your case: write down the conditions right away. Eye injuries, contacts, fatigue, wildfire smoke irritation, flashing lights, passing traffic, and uneven footing can all affect what the officer thought they saw. If there was a crash, that can also spill into an injury claim, because HGN observations may influence fault arguments, insurance negotiations, and whether the other side tries to paint you as impaired.