How the Police Test BAC: FAQ

At a traffic stop, law enforcement officers might use field sobriety tests. These tests can give them some indication as to whether you’re impaired. But they don’t measure your exact level of impairment. That’s where BAC tests come in handy.

This article answers frequently asked questions about BAC tests. It explains the methods used to measure blood alcohol levels and how Breathalyzer tests work. You’ll also learn about the accuracy of BAC tests and whether it’s possible to challenge Breathalyzer results in court.

If a police test shows you’re over the legal limit for alcohol, you’ll face an arrest for driving while intoxicated (DWI). This is when the advice of a nearby DUI lawyer can be valuable. Get help for a DUI case promptly.

What methods are used to measure blood alcohol concentration (BAC)?

Your blood alcohol content can be measured in multiple ways. The police can use limited types of BAC tests at a traffic stop. More advanced tests are available at the police station after they bring in a driver suspected of drunk driving.

To estimate your blood alcohol concentration, police can use samples of:

• Blood
• Breath
• Urine
• Saliva
• Hair follicles

Police officers typically only use breath analysis and blood screenings for gathering evidence. Measuring a person’s BAC with a preliminary alcohol screening (PAS) device or a Breathalyzer is the most common option. Blood screening is considered more invasive and is used less often than breath analysis. Blood testing is mainly employed after a serious accident or when a suspect refuses a breath test.

How do Breathalyzer tests work?

Breathalyzers, or breath analysis machines, work by analyzing the alcohol content of exhaled vapor in your breath. Any alcohol from the vapors in your breath reacts with a chemical solution in the machine, creating a color change in the machine’s reading. A computer translates the data into the BAC measurement used to determine the level of alcohol in your bloodstream (for example, 0.08 percent BAC).

How accurate are Breathalyzer tests?

While not as accurate as blood tests, most courts consider Breathalyzer tests acceptably accurate as tools for gathering evidence and determining impairment.

However, some independent studies have determined that breath readings can vary by 15% from actual BAC levels as measured by a blood draw. Some courts have even thrown out Breathalyzer results, calling into question the reliability of the machines.

For example, in 1988, a New Jersey court cited the following scientific evidence questioning breath tests:

• High readings for 14 percent of the population due to design flaws
• Variance in results based on the temperature of the machine itself
• Different results from the varying body temperatures of test subjects
• Variances in the presence of hematocrit in the blood, affecting test results

Though technology has changed since 1988, many potential flaws still apply to DUI cases today. These flaws also include human error. Using an improperly calibrated machine can hamper the accuracy of Breathalyzer test results. Once again, New Jersey serves as an example. In 2024, the state found that thousands of breath tests from 2008 to 2016 were improperly calibrated.

Can you beat a Breathalyzer test if you’re intoxicated?

No, you can’t beat a Breathalyzer test if you’re under the influence of alcohol. Some popular methods are commonly believed to help you fool a Breathalyzer test.

These methods include:

• Ingesting breath mints
• Using mouthwash
• Sucking on pennies

These methods, however, have been shown not to work. Mouthwash, which often contains alcohol, may actually raise your BAC.

Plus, the presence of alcohol isn’t the only possible cause of impairment while driving. Prescription drugs, illegal drugs, or recreational marijuana won’t show up in a BAC breath test. So even if you pass a breath test, police may still arrest you on suspicion that you’re under the influence of something else.

Is it possible to successfully challenge Breathalyzer results in court?

Yes, it’s possible to challenge Breathalyzer test results in court successfully. Breathalyzer machines must be tested routinely to make sure they’re properly calibrated.

A skilled DUI lawyer can help evaluate your defense options. They may look into the maintenance records of the device used to test you and otherwise determine the validity of the evidence.

How accurate are the small BAC–testing devices sold to consumers?

The accuracy of the portable car BAC devices sold to consumers depends on many factors. There is a wide range of products available. Some are more accurate than others. It’s best to assume the readings on these devices are a little off and may not be used as a defense to DUI charges.

Saliva alcohol test strips are among the most accurate consumer BAC tests. This is because the amount of alcohol in your bloodstream closely corresponds to the alcohol concentration in your saliva.

Can a police officer force me to give a blood sample instead?

Yes, in some cases they can require a blood test. You can refuse field sobriety tests and breath tests, though you’d face the consequences of refusal under implied consent laws. Yet, some situations allow the police to conduct a more invasive blood test, even when you refuse.

The police will usually need a warrant to conduct a forced blood test. Blood testing qualifies as a search under the Fourth Amendment of the Constitution. You have a right to protection against unreasonable searches. Because blood tests are so invasive, the reasonability is subject to high scrutiny.

If police have probable cause that you were drunk or drugged while driving, they may successfully get a warrant for a mandatory chemical test. Probable cause is a higher legal standard than reasonable suspicion. Police only need reasonable suspicion to pull you over for a DUI stop.

There are few exceptions, or “exigent circumstances,” to this warrant requirement. The main scenario is in a DUI accident response. If a driver is unconscious after a crash, the police may gather a blood sample in the course of the driver’s medical treatment.

Was Your BAC Test Accurate? A Defense Attorney Will Help Challenge the Evidence

Understanding the science behind a blood or breath sample result can be challenging. A drunk driving conviction can carry serious consequences, including jail time, the loss of driving privileges, and a driver’s license suspension.

Contact a DUI defense attorney in your area to learn more about DUI laws and your options following a DUI arrest. A defense lawyer can help you with your DUI defense, whether you’re a first-time offender or you’ve had a previous DUI conviction.