Groups » How Reliable Are DUI Tests?

Virtually everyone acknowledges that drunk driving is a problem these days. Horror stories abound in which a driver drinks far too much, gets behind the wheel, and causes an accident resulting in bodily injury and/or death to others. Yet there are also stories in which a driver has perhaps one – maybe two – drinks over an extended period of time, is later stopped because a brake light is out, is given a field sobriety test, and arrested for driving under the influence.

These stories make many wonder: Are the various tests used to determine blood alcohol level reliable, particularly in close cases?

To answer this question, we first need to understand a little bit about the different types of tests that are administered. Blood alcohol tests are generally divided into two types: (a) physical dexterity tests given in the field and (b) laboratory or chemical analysis tests.

Field Sobriety Tests

Field sobriety tests of one type or another have been given in the field by police officers for more than 75 years. Many early tests were inherently unreliable; the failure rate of people who were not under the influence of drugs or alcohol was quite high. Responding to criticism, the National Highway Traffic Safety Administration (NHTSA), part of the U.S. Department of Transportation, developed a set of standardized field sobriety tests (SFSTs) in the late 1970s, in order that police officers across the nation could better determine if a driver was over the legal limit of blood alcohol content.

Types of Standardized Field Sobriety Tests

Three primary physical tests were designed by the NHTSA:

• The one-leg stand, in which the officer will ask the driver to stand in front of the officer and lift one leg off the ground, usually for 30 seconds. During that time period, the officer observes the driver’s relative sense of balance.

• The walk and turn, in which the officer asks the subject to take a series of steps in a straight line, placing one foot directly in front of the other. After a few steps, the officer tells the subject to turn, and walk back the same way.

• The horizontal gaze nystagmus test, in which the officer asks the driver to look at an object – ordinarily a pen – that the officer holds close to the driver’s face. Then the officer tells the driver to watch the object as the officer moves it from side to side. The officer observes the driver’s eyes, to determine how “smoothly” the driver moves his or her eyes. If the eye movement is jerky, this is ordinarily a sign that the driver may be impaired.

Reliability of Standardized Field Sobriety Tests

Even after the effort by the NHTSA to develop standardized tests, the accuracy of these field sobriety tests continue to be questioned. While some organizations, such as the American Auto Association, argue that the tests are 90% accurate, others disagree.

For example, in a study from Scotland, which uses virtually identical SFSTs, 25 police surgeons attended a full day training program to learn about the tests. After the training, almost half the doctors expressed concerns about their accuracy. And these were police doctors!

Other critics point out that in spite of the fact that the field sobriety test consists of a battery of intricate eye movements, balance, vision, and neurological function, the tests must be administered by police officers who have no formal education in any of these areas and who have only received a few days training. The officers must do all this, of course, on the side of the road, usually at night!

The horizontal gaze test is particularly problematic, since people who have no alcohol in their system often fail it. Moreover, according to a prominent ophthalmology textbook, "meticulous history-taking and drug-screening blood studies are often essential in evaluating patients with nystagmus.”

Laboratory Tests

Blood Tests

Blood tests are thought by many to be the most accurate method of testing for impairment. After all, if one wants to know the level of alcohol in the blood, wouldn’t it make sense to test the blood? Indeed, but since blood tests are administered by humans and subject to various procedural issues, they too can be fraught with problems. For example, in one report from a few years ago, it was determined that an Orange County, California crime lab made mistakes in more than 2,200 DUI cases in a single year.

Particularly problematic can be blood tests administered after an accident in which the driver has been injured. If the driver has lost a significant amount of blood, the tests may not be accurate. Other factors that can compromise the accuracy of blood tests include:

If the person being tested suffers from anemia.
If alcohol swabs are used to cleanse the area where the testing needle is inserted in the driver’s arm.
If blood drawn at the accident scene is put into a standard emergency kit that contains chemicals to preserve the blood—inaccurate readings can result.
If ER doctors request that blood samples be centrifuged for immediate analysis and treatment.
If the blood is drawn after first responders or other medical care providers have begun to administer drugs or other foreign substances into the driver’s body following an accident.
If blood plasma or amphetamines are used to stimulate the heart and lungs, this can obscure the actual alcohol level in the blood or the absorption rate for alcohol.

Breath Tests

Breath tests using handheld devices – so-called “breathalyzers” and “preliminary breath testers” (PBTs) – are the most common tests used by police officers to “test” levels of alcohol in a driver’s blood. They are popular with police because they are lightweight, portable, and easy to administer.

The primary difficulty with the tests is that they don’t actually test for alcohol levels in the blood, but rather estimate it based on the contents of the driver’s breath. The machines must be carefully calibrated. Body temperature, body weight, and respiration rate can affect the test’s outcome.

Some of the devices test for the presence of ethyl molecules in the breath. Critics point out that as many as 100 compounds can be found in human breath at any one time and that 70 to 80 percent of them contain methyl molecules.

Urine Tests

Most experts say that among the three common chemical tests for blood alcohol levels – blood testing, breath testing and urine testing – urine testing is the least reliable. They say:

The percent of alcohol in the urine is not necessarily the same as in a person’s blood.
Since urine is stored in the bladder and remains there until the bladder is emptied, the bladder’s contents represent a shifting composite, providing much less real information than required to show that a person is impaired beyond a reasonable doubt. One cautionary point: the shifting composite argument can work both for and against a driver charged with a DUI.
Like other medical tests, they are subject to error.


A DUI conviction can have grave consequences to a driver. Loss of one’s driver’s license, increased auto insurance premiums, lost time from work—the list goes on. Particularly where the test indicates that the driver is barely over “the legal limit,” it may be advantageous to fight the charges in an aggressive manner. This isn’t something that can typically be done on one’s own, however. One should consult an experienced, knowledgeable attorney.

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