What percentage accuracy does Horizontal Gaze Nystagmus (HGN) achieve by itself?

The Horizontal Gaze Nystagmus (HGN) test is one of the three standardized field sobriety tests used to assess a driver's level of impairment due to alcohol. When evaluated properly, studies have shown that HGN testing has demonstrated a significant accuracy rate in identifying individuals with a blood alcohol concentration (BAC) of 0.08% or higher.

Data indicates that the accuracy of the HGN test alone achieves approximately 77%. This figure reflects the high correlation between the presence of nystagmus and higher levels of impairment caused by alcohol consumption. Nystagmus is defined as an involuntary eye movement that becomes more pronounced as a person’s BAC increases, making it a reliable indicator when executed in controlled conditions.

Understanding this accuracy is critical for law enforcement, as it informs their decision-making process during sobriety checks. The use of this statistic can also be beneficial in legal contexts, both for enforcing DUI laws and for investigating possible defenses.