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Chemical & Field Sobriety Tests

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Chemical and field sobriety tests are one component of the stop, arrest and/or prosecution of a person suspected of driving under the influence of drugs or alcohol. If you are charged with DUI, contact a drunk driving attorney immediately. If the chemical and/or field sobriety test results were unlawfully or improperly administered, or if there is other evidence to suggest the results are not accurate, your lawyer may be able to have the results and possibly the charges against you dismissed.

Field Sobriety Tests

Field sobriety tests are usually administered roadside by a police officer who suspects a motorist of DUI. Field sobriety tests do not measure blood alcohol content (BAC); they can only suggest that a driver is impaired. Field sobriety test results are primarily used to establish probable cause for a DUI arrest, and by prosecutors at trial to support their case against you. In many states field sobriety tests are voluntary and may be declined. Field sobriety tests should not be confused with chemical tests, which you are legally required to take.

Horizontal Gaze Nystagmus (HGN)

Of the standardized field sobriety tests, the HGN test is believed the most reliable. HGN is an involuntary jerking or bouncing of the eyeball that can occur for a variety of reasons, including consumption of alcohol. During the HGN test, the suspect follows a small object side to side with his or her eyes while the officer looks for these indicators of impairment:

  • The eye can't follow a moving object smoothly
  • Jerking is distinct when the eye is at maximum deviation
  • Angle of onset of the jerking is prior to 45 degrees

The most common field sobriety tests recommended by the National Highway Transportation Safety Administration (NHTSA) are a three-test battery of standardized field sobriety tests (SFTS) which includes the horizontal gaze nystagmus, walk and turn, and one-leg stand (see sidebars). Even if the SFTS tests are performed correctly by a specially trained officer, they are at best 91 percent reliable at predicting a BAC of .08 or higher. Other studies have shown the tests to be even less reliable, especially in persons with a BAC under .08.


If you "fail" the standardized field sobriety tests, an experienced DUI attorney may object to the results by arguing:

  • The tests are not scientific
  • The tests are not reliable
  • The officer was not properly trained to administer the tests
  • The officer did not properly administer the tests

Other problems associated with the SFTS include:

  • Environmental factors (e.g., poor weather, blinding traffic)
  • Anxiety, intimidation
  • Illness (e.g., multiple sclerosis, brain tumor, inner ear disorders)
  • Physical injury and disability (e.g., obesity, knee or hip problem)
  • Medication (e.g., sedatives, seizure medications)
  • Age

If you failed standardized field sobriety tests, your DUI attorney will evaluate the facts and circumstances of your case and put forth the appropriate challenges to the test results.

Walk and Turn and One-Leg Stand

Walk and turn and one-leg stand are part of the standardized field sobriety tests and are considered divided attention tests. During walk and turn, the suspect is asked to walk a straight line, heel to toe. If the officer observes two or more of the following, the suspect "fails" the test:

  • Unable to keep balance while listening to the instructions
  • Begins before the instructions are finished
  • Stops while walking to regain balance
  • Does not touch heel to toe
  • Steps off the line
  • Uses arms to balance
  • Makes an improper turn
  • Takes an incorrect number of steps

During one-leg stand, the subject stands on one foot for thirty seconds while counting aloud by the thousands. If the officer observes two or more of the following, the subject "fails" this test:

  • Swaying while balancing
  • Using arms to balance
  • Hopping to maintain balance
  • Putting the foot down

Chemical Tests

In every state, if a driver has a BAC level at or above the legal limit (.08 for drivers over 21 years of age and less for underage and commercial drivers), a violation has occurred without regard to other evidence of intoxication or sobriety. Chemical tests, including breath, blood and urine tests, are used to measure the BAC of a DUI suspect. If you are stopped lawfully and then suspected of DUI it is against the law to refuse to take a chemical test. This is called implied consent, and those who violate implied consent laws usually have their licenses suspended. If you are arrested, you have the right to post bail. Bail bonds are obtained through a bail bondsman. (Bail for DUI cost varies.)

Breath Tests. Breath analyzer tests such as the Breathalyzer are conducted roadside (usually a preliminary breath test or PBT) or at the police station to provide an estimate of your BAC. PBT results might be used to help establish probable cause for arrest and further chemical testing, but might not be admissible in court. Common challenges to breath analyzer test results include:

  • Police failed to conduct a continuous observation period (usually 15-20 minutes) prior to administering the test
  • The machine was not properly calibrated
  • The test was not performed correctly
  • The operator was not licensed or the license expired
  • Sampling error
  • Medical conditions (e.g., GERD)
  • Mouthwash, cough drops, asthma medication, paints, nail polish or other substances interfered with the results

Blood and Urine Tests. Blood and urine tests can be used to verify the result of a breath test, or on their own to establish BAC (blood test) or drugs (urine test). Blood tests are more accurate than urine tests. A skilled DUI attorney may object to blood or urine tests on the following grounds:

  • Lab error
  • Technician was not qualified to perform the test
  • Test performed incorrectly (e.g., failed to add anti-coagulant or preservative)
  • Diet
  • Medication
  • Break in chain of custody
  • Sample was not properly preserved (e.g., refrigerated)
  • Contamination of the sample

Your DUI lawyer will conduct a thorough investigation of the testing procedures and may request the quality control testing records, chain of custody records, certifications and more.

DUI Lawyers

Of course, there may be many more factors to consider with respect to chemical and SFTS testing that might affect your DUI/DWI case. To learn more, schedule a consultation with a DUI attorney who will explain your state's DUI laws, as well as how they will apply to the facts of your case. Considering the severity of most penalties for DUI — legal fees, loss of license, DUI jail time, etc. — it is imperative you seek counsel if you are charged with a DUI or DWI. Whether you are facing a first offense or are a repeat offender, a DUI lawyer can help you to understand your rights and limit your liability.