A Brief Guide to the Different Types of Field Sobriety Tests

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Why Are Field Sobriety Tests Conducted?

Field sobriety tests (FSTs) are used to enforce a state’s driving while intoxicated (DWI) laws. They are often conducted before a breath test or other chemical tests such as urine and blood tests. A law enforcement officer can use these tests to evaluate whether a person suspected of impaired driving is under the influence of alcohol or other controlled substances.

The National Highway Traffic Safety Administration recognizes three standardized field sobriety tests. They are administered by law enforcement officials such as police officers to:

  • Assess if a motorist is intoxicated.

  • Examine the driver’s balance and coordination.

  • Test the ability of the driver to split attention between multiple activities.

Signs of Intoxication


There are specific signs that a law enforcement official looks for to determine if someone is intoxicated. The following are some of the telltale signs that someone may be drunk and fail a field sobriety test:

  • Absence of inhibition
  • Being too chatty or joyful
  • Bloodshot eyes
  • Difficulty in maintaining balance
  • Flushing/redness of the face
  • Slurred voice or other communication issues

You may be asked to participate in a field sobriety test if you are stopped at DWI checkpoints in Missouri and exhibit any of the above signs of intoxication.

Standardized Field Sobriety Tests (SFSTs)

As explained above, some FSTs are recognized by the National Highway Traffic Safety Administration (NHTSA). They include the following: 

  • Horizontal Gaze Nystagmus (HGN) Test

  • Walk-and-Turn Test

  • One-Leg Stand Test

These tests examine and evaluate indicators of impairment such as eye movements, balance, and ability to follow instructions. 

Horizontal Gaze Nystagmus Test

Nystagmus refers to involuntary jerking eye movements when your eyes move from side to side, up-down, or in a rotatory motion. When drunk, nystagmus can become accentuated. People under the influence of alcohol or drugs may have trouble visually monitoring a moving object.

During this test, the officer horizontally moves an object, such as a pen or flashlight, and asks you to follow it with your eyes. The officer will look for three signs of impairment: 

  • When the eyes are “at maximum deviation,” they twitch sharply.

  • You are unable to maintain a steady track of the moving item.

  • Your eyes begin to twitch within 45 degrees of the center.

One-Leg Stand Test

The officer will advise you to stand with one foot lifted roughly six inches parallel to the ground. At the same time, the officer will ask you to count loudly until told to stop. The officer looks for four signs of impairment: 

  • Swaying

  • Balancing using the arms

  • Hoping to preserve equilibrium

  • Inability to balance, for example, putting a foot down

Walk-And-Turn Test

The officer instructs you to take nine heel-to-toe steps down a straight line. You then turn on one foot and return nine steps in the other direction. The officer looks for seven symptoms of impairment during the exam. Disqualification occurs when you: 

  • Begin before the instructions have been completed

  • Cannot maintain balance while listening to instructions

  • Do not have heel-to-toe contact

  • While turning, lose equilibrium

  • Stop walking to recover equilibrium

  • Take the wrong number of steps

  • Use your arms to balance

Other Non-Standardized Tests

Police officers occasionally use non-standardized field sobriety tests to charge drivers with DWI. Examples of other tests include: 

  • Romberg Balance Test

  • Asking suspects to write or recite the alphabet to assess for impairment. 

  • Instructing individuals to count backward. 

These tests have not been scientifically proven to assess impairment. 

Chemical Tests

Missouri provides a variety of chemical testing alternatives to field sobriety tests. Breathalyzers and blood tests are common alternatives to determine the BAC levels in breath and blood samples, respectively. Urine tests are less prevalent and less reliable in general than breath and blood tests.

Breathalyzer Test

This test estimates a person’s blood alcohol concentration (BAC) by analyzing their breath sample. The legal alcohol limit in Missouri is 0.08% for drivers aged 21 or above.

Blood Test

Blood tests may be used when a breathalyzer is unavailable, or drug impairment is suspected. They are conducted to determine the presence of alcohol or drugs in a person’s system. 

Accuracy of Field Sobriety Tests in Determining Impairment


Though the results of a field sobriety test are acceptable as evidence in court, neither the standardized nor the non-standardized tests are entirely accurate. Each test has variables that can affect it. In fact, several incidents have occurred where people “failed” a test while being perfectly sober and in control of their mental faculties. 

Effect of Medical Conditions on the Performance of Field Sobriety Tests

A pre-existing health issue is one of the primary reasons for an inability to complete the field sobriety tests successfully. Factors such as injury, deafness, or eye issues, may contribute to erroneous testing. The stress of the situation may also lead you to appear and act anxious, which may be misinterpreted as a sign of intoxication. 

Challenging Field Sobriety Tests in Court

These tests are subjective. It is possible to challenge them in court to weaken the prosecution’s case. For example, police officers undergo a brief training program to administer these tests. Failed tests can result from incorrect instructions, missing steps, or confusion. A fair evaluation requires adequate observation of the test participant. Many factors can contribute to failed tests. They include the following: 

  • Age

  • Anxiousness 

  • Clothing

  • Flashing police lights 

  • Illness

  • Lack of illumination 

  • Medical issues

  • Medicines

  • Physical injuries

  • Rapid traffic

  • Shoes

  • Temperature or weather 

  • Uneven terrain 

  • Weight

Adequate training is crucial for administering sobriety tests. You can weaken the prosecution’s case by proving that the administering officer was not adequately trained. Or you could show that they failed to follow standardized procedures.

Do You Need Help Challenging a Sobriety Test?

Do you need to challenge a field sobriety test? Or you may have questions surrounding the Missouri DWI laws. Our skilled and knowledgeable lawyers at the Ozarks DWI Law Clinic are here to help you do just that.

We have a successful track record challenging sobriety tests. Just because you have been charged with a DWI offense and performed a field sobriety test does not mean the test will stand up to legal scrutiny in court. Let our experienced lawyers help you navigate the Missouri legal system. Contact our DWI Attorney Ozark, MO, for a free case consultation today!