What tests are administered to determine driving under the influence?

The Pennsylvania state legislature associates DUI convictions with criminal penalties, including jail time, fines and driving restrictions. Enforcement of DUI and related offenses rests with authorities on the ground. Drivers whom authorities suspect of drunken driving may be asked to participate in various sobriety tests.

Field sobriety tests

The first set of tests that may be administered comprise a field sobriety test. The National Highway Traffic and Safety Administration endorses what is called the Standardized Field Sobriety Test, which consists of three distinct tests:

  • The horizontal gaze nystagmus - The horizontal gaze nystagmus test requires drivers to follow with their eyes an object that the authority manually moves from side to side. This unusual term, nystagmus, refers to a natural, involuntary jerking of the eye that occurs when the eye gazes to one side or the other. When the participant is under the influence of alcohol, the jerking motion is exaggerated.
  • The walk and turn - The walk and turn test requires participants to walk nine steps along a straight path, after which they must turn on one foot and do the same in the opposite direction.
  • The one-leg stand - The one-leg stand test requires participants to remain standing on one foot, with the other elevated from the ground, while counting to 30.

While law enforcement officers use the above tests as part of the determination process, people with health problems may have trouble passing them which can lead to false DUI arrest.

Chemical tests

The second set of tests that authorities generally administer consists of the chemical tests. These tests are designed to measure blood alcohol content, which represents the percentage of alcohol in the blood. The first utilizes a breathalyzer device, which indirectly measures BAC by way of the amount of alcohol in the participant's breath when exhaling. In the state of Pennsylvania, the threshold at which BAC becomes illegal is 0.08. Given that the measurement of BAC through a breathalyzer is an indirect test, it is typically supplemented with an actual blood test, using a sample of the participant's blood.

Tests are not foolproof

Any tests used to determine a person's BAC can be subject to error. Any conclusions inferred from field sobriety tests, for example, are at least partially dependent on the officer's interpretations of his observations. Even chemical tests of BAC are vulnerable to error. Breath test results may be incapable of distinguishing alcohol in the bloodstream due to an alcoholic beverage from alcohol on the breath due to a mouthwash product. Blood samples may yield different results when evaluated for a second time at a private laboratory. For these reasons, drivers in Pennsylvania charged with a DUI may wish to consult with a criminal defense attorney.