End-position nystagmus as an indicator of ethanol intoxication.

The Horizontal Gaze Nystagmus test is used by law enforcement agencies in the United States to determine whether drivers are intoxicated. It has a high baseline error and a dose/response relationship that varies greatly according to whether the subject's blood alcohol concentration is rising or falling. Confusion exists among practitioners of the test about whether it quantifies alcohol concentration or evaluates impairment. Fatigue exacerbates one component of the HGN test, end-position nystagmus. Video tapes recorded by cameras in police vehicles revealed that police officers rarely comply with the minimum requirements of the nystagmus examination procedures for which they were trained and certified.