DUI/DWI Testing Procedures

An overview of the tests that police officers use to determine if a driver is intoxicated

By Canaan Suitt, J.D. | Last updated on November 8, 2023 Featuring practical insights from contributing attorneys Stephen P. Pfeiffer and Mark Thiessen

Use these links to jump to different sections:

According to the National Highway Traffic Safety Administration (NHTSA), approximately 37 people die in drunk driving-related car crashes every day. Hundreds more are injured. Intoxicated driving accounts for roughly a third of all fatal car crashes in the United States annually.

Because of the dangers drunk driving poses to public safety, the consequences of a DUI conviction—even on a first offense—are severe. And police officers take enforcement very seriously.

This article gives a quick overview of when police officers can pull drivers over for impairment, and the testing procedures officers use at traffic stops and following a DUI arrest.

If you are facing DUI charges, it’s imperative to speak with a DUI/DWI defense attorney as soon as possible. A defense attorney’s job is to ensure that law enforcement followed correct procedures, protect your legal rights, and mitigate the negative consequences of a DUI/DWI as much as possible.

When Can a Police Officer Pull You Over for Impairment?

Law enforcement officers can’t stop drivers simply because they feel like it. They must have a reasonable suspicion that some illegal activity is going on to stop and temporarily detain a driver at a traffic stop.

“There are two ways that you can get pulled over,” says Mark Thiessen, a criminal defense lawyer in Houston, Texas, who specializes in the law and science of DUI cases. Either:

  • You commit a traffic violation. There are various traffic violations—speeding, swerving, failing to use your signal light, or having a lamp out. When a police officer has reasonable suspicion of illegal activity based on these sorts of factors, they can pull the driver over.
  • Police engage in community caretaking. Police officers have community caretaking responsibilities. If somebody calls the police and says they’re worried about a driver, or a driver is passed out and a bystander thinks they’re going through a health emergency, the police will pull the driver over because they believe they are in distress and might need help.

I think it’s absolutely imperative, if someone truly cares about their record and future, to hire a competent DUI attorney if they’re dealing with DUI charges. There are just too many nuances in this area of law to go at it alone. To give yourself the best chance, you want to have somebody who focuses on this area of law.

Stephen P. Pfeiffer

When Can a Police Officer Detain You For Testing?

“First, police need a reasonable suspicion to pull you over. Then, once they stop and detain you, they need reasonable suspicion to continue the detention beyond just the traffic stop,” says Thiessen.

“There are many kinds of signs of intoxication that would allow the police to prolong the detention from an initial traffic stop or community caretaking to saying: ‘Hey, I think I need you to get out of the car and do some tests.’”

Common signs of intoxication include:

  • Slurred speech;
  • Bloodshot eyes;
  • Open bottles or cans of alcohol in the car;
  • A heavy odor of alcohol coming from the car or person; and
  • Lack of coordination or ability to respond to questions coherently.

The officer may also pick up on contextual clues such as the time of day and if there are bars, breweries, or special events involving alcohol in the area.

If there are no signs of intoxication, the police officer can issue a ticket for any traffic violations the driver committed but should let them go at that point. “Either: ‘Hey, we checked you out, and you’re fine,’ or ‘Hey, here’s a ticket; go on your way,’” explains Thiessen. However, if the officer sees signs of intoxication, they will likely ask the driver to take some tests to verify if they’re under the influence of alcohol.

First, police need a reasonable suspicion to pull you over. Then, once they stop and detain you, they need reasonable suspicion to continue the detention beyond just the traffic stop. There are many kinds of signs of intoxication that would allow the police to prolong the detention from an initial traffic stop or community caretaking to saying: ‘Hey, I think I need you to get out of the car and do some tests.’

Mark Thiessen

Standard Field Sobriety Tests (FSTs)

Police officers are trained to administer several types of tests to gauge a driver’s impairment. Known as the standard field sobriety test (SFST), it includes three procedures:

  • Horizontal gaze nystagmus test. In the HGN test, the police officer looks for involuntary jerking of the eyes (“nystagmus”) by asking the driver to follow an object back and forth with their eyes. Officers often use a pen placed approximately 12 inches away. The basic idea behind the test is that involuntary jerking of the eyes is less controlled and more exaggerated when a person is under the influence of alcohol or drugs.
  • Walk and turn test. In the WAT test, a police officer asks the person to take nine heel-to-toe steps in a straight line, turn around on one foot at the end and walk back the same way. The idea is that sober individuals have no problem performing these actions. If a person takes an incorrect number of steps or can’t walk in a straight line, it indicates they might be intoxicated.
  • One leg stand test. In the OLS test, a police officer asks the person to stand with one leg six inches off the ground and hold the position for 30 seconds. If the person sways, hops, or has to use their arms to balance, officers will use those indications as clues that the person is impaired.

Failing the SFST gives the police officer probable cause for a DUI arrest. That being said, each of the tests included in the SFST is subject to challenge by DUI defense attorneys. Learn more about each type of field sobriety test and DUI defense strategies to contest their results.

Breathalyzer Testing

Police officers are also trained to administer breathalyzers or breath tests. The person breathes into the breathalyzer, and the machine provides a measure of the person’s blood alcohol concentration (BAC).

Police officers might ask you to blow into a portable breathalyzer at the DUI arrest point or wait until you’re back at the police station for further testing.

In most states, the BAC legal limit for drivers over 21 years old is 0.08 percent. According to the NHTSA, the effects of a 0.08 BAC include:

  • Poor muscle coordination;
  • Difficulty detecting danger; and
  • Impaired judgment, self-control, reasoning, and memory.

BAC legal limits are lower for drivers under 21 (typically 0.02) and drivers with a commercial driver’s license (typically 0.04).

A benefit of breath tests is that they give a quick read of a person’s BAC. The problem is that they aren’t infallible. Many things can skew their results, such as trace amounts of alcohol in your mouth or esophagus. DUI lawyers frequently challenge breath test results in court.

Learn more about BAC levels and find answers to frequently asked questions about breathalyzer tests, including whether it makes sense to refuse one. 

Blood Tests and Urine Tests

Following a DUI arrest, law enforcement will take you to the police station for booking. Once there, further BAC chemical tests may be administered. For example, you may be asked to undergo one of the following:

  • Blood test;
  • Urine test;
  • Breath test (possibly in addition to the portable breathalyzer test at the roadside).

Thiessen explains that in Texas, drivers who have been arrested for a DUI/DWI have the option of consenting to either a breath test or a blood test at the police station. In Virginia and other states, by contrast, which type of test to use “is not the defendant’s election; it’s up to the officer,” says Stephen P. Pfeiffer, a criminal defense attorney and DUI law expert in Virginia Beach. Typically, the arresting officer will only go for a blood test if the breath test machine is unavailable or the defendant is incapacitated or has a medical condition that makes using the breath test untenable.

Blood and urine tests are submitted to toxicology labs for testing, and it often takes months to get results back, whereas breath test results are immediate. And in addition to testing your alcohol levels, urine and blood samples can also pick up on the presence of medications or controlled substances in your system.

Drug Recognition Evaluation (DRE)

A drug recognition evaluator (DRE) is a law enforcement officer trained to recognize impairment from drugs other than alcohol—for example, the impairment from marijuana or prescription drugs.

DREs undergo training through the NHTSA-sponsored Drug Evaluation and Classification Program (DEC). 29 states, plus the District of Columbia (DC), participate in the DEC program.

DREs administer a standardized 12-step test to evaluate impairment, including a breath alcohol test, an interview with the arresting police officer, and testing the person’s vital signs.

Get an Experienced DUI Defense Lawyer

One of the main areas of DUI defense is contesting the administration, validity, and results of field sobriety and BAC tests. To take advantage of these DUI defense strategies, it’s essential to consult with an experienced DUI attorney who understands the DUI law and procedures in your state.

“A DUI conviction will affect your driver’s license and all areas of your life,” cautions Thiessen. It’s not something to fight on your own and without legal experience.

“I think it’s absolutely imperative, if someone truly cares about their record and future, to hire a competent DUI attorney if they’re dealing with DUI charges,” adds Pfeiffer. “There are just too many nuances in this area of law to go at it alone. To give yourself the best chance, you want to have somebody who focuses on this area of law.”

So: If you have been arrested for a DUI and are facing DUI charges, consult a DUI defense attorney as soon as possible. To find an experienced DUI attorney in your area, use the Super Lawyers directory to search by topic and jurisdiction.

What do I do next?

Enter your location below to get connected with a qualified attorney today.

Find top lawyers with confidence

The Super Lawyers patented selection process is peer influenced and research driven, selecting the top 5% of attorneys to the Super Lawyers lists each year. We know lawyers and make it easy to connect with them.

Find a lawyer near you