When a law enforcement officer asks me to follow a pen light, what is he looking for?

This examination is known as the horizontal gaze nystagmus (HGN) test. Nystagmus is the medical term used to describe a specific eye movement. The stability of your eyes while tracking the pen and the point at which they start moving irregularly indicate the alcohol level in your bloodstream. The officer is trying to determine if the angle of your eyes is less than 45 degrees. An angle below 45 degrees generally suggests a blood-alcohol level of .05% or higher. The horizontal gaze nystagmus test is relatively new and is not universally accepted in many states. Numerous medical professionals do not consider this test to be reliable. Additionally, it is possible that some law enforcement officers conducting this test may not be qualified to administer it. An untrained officer may struggle to position the pen correctly and therefore misinterpret nystagmus.