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Field Sobriety Test

Field sobriety test can be performed when a police officer suspect that a driver is under the influence of alcohol. The test includes:

1. Horizontal Gaze Test

An officer moves the finger horizontally left and right in front of the persons eyes. Rapid involuntary eye movements (nystagmus) during the test are a reliable sign of alcohol intoxication and speak for blood alcohol concentration (BAC) above 0.1 g/100 mL blood [1].

2. Vertical Nystagmus

Vertical nystagmus speaks for severe alcohol intoxication [1]. During the test, a person should not look at flashing lights or moving objects in order to prevent “optokinetic nystagmus” [1]. It is not likely that a law-enforcement officer would confuse the horizontal gaze nystagmus with nystagmus due to other causes of rapid eye movements, such as natural nystagmus, eye strain, fatigue, inner ear disease, brain tumor or damage. From the angle at which the nystagmus appears, BAC may be roughly estimated.

Other Field Tests

  • Walking along a straight line
  • Standing on one leg

The field sobriety test does not give reliable information about how intoxicated the tested person may be. When the police officer suspects the person is intoxicated, he or she will usually continue with a breath alcohol test.

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