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New iPhone App. Used to Capture Horizontal Gaze Nystagmus May Expose Errors in the Seminal Field Sobriety Test

A new iPhone app called Breathaleyes uses the iPhone camera to capture Horizontal Gaze Nystagmus, the common field sobriety test that looks for eye movements caused by alcohol. To measure its accuracy, officers from the Huntsville Police Department put the app to the test. Three volunteers drank three beers in an hour. Thirty minutes later two volunteers blew a between .05 and .08 BAC and one volunteers blew a .10. Compared to the relative blood alcohol content determined by the Breathaleyes HGN test, each volunteer had a BAC over .08 with two almost double the legal limit.

A HPD officer concluded that he “would take [the results] with a grain of salt,” that he “wouldn’t trust the app.” The app or the HGN test itself? The officers never tested the app results versus a non-computerized HGN test, only a calibrated breathalyzer. Considering those police training manuals state that field sobriety tests are only accurate 77% of the time and that the HGN test can be undermined by involuntary eye movements caused by flashing lights or sudden changes in heat or cold, this crucial comparison is a telling omission.

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