DUI Testing: Breath, Blood, and Warrants

Do the police need a warrant to take your blood? Can you be prosecuted for refusing a test? Learn about the U.S. Supreme Court's rules on DUI testing.

By , Attorney · UC Law San Francisco

In 2016, the U.S. Supreme Court held that laws that make it a crime to refuse a blood test after a DUI arrest are unconstitutional. The Court did say, though, that it's fine for a law to make it a crime to refuse a post-arrest breath test.

The Supreme Court also found that an officer has to get a warrant in order to require a driver who doesn't consent to a blood test to provide a blood sample. The officer does not, however, need a warrant in order to demand that the driver take a breath test.

In 2019, the Court added some nuance to the rule that an officer needs a warrant to take a blood sample that the motorist hasn't agreed to. Here's what they said: If a driver is unconscious and thus can't be given a breath test, officers almost never need a warrant to collect a blood sample for testing.

Implied Consent

"Implied consent" laws impose penalties on drivers who refuse to submit to chemical testing when there's a basis to believe they've just been driving while intoxicated. All 50 states have laws that require motorists who've been arrested or detained on suspicion of drunk driving to consent to blood alcohol concentration (BAC) testing.

BAC testing normally happens through analysis of a driver's breath or blood. Lots of states' implied consent laws either say that officers should administer breath tests in DUI cases or give the driver some kind of choice between breath and blood.

Penalties for Refusing

An implied consent law makes cooperating with BAC testing a condition of the privilege of driving. Don't submit to testing in appropriate circumstances, and you lose the privilege—at least for some period of time. And, should you go to trial, the prosecution can present the fact that you refused testing as evidence that you knew you had had too much to drink.

Many states' laws provide for noncriminal (often called "administrative") penalties for refusing to undergo DUI testing. But some go the extra step of making refusal a crime.

Breath, Blood, and Warrants

In the 2016 case, Birchfield v. North Dakota, the U.S. Supreme Court made clear that there's a big difference between having to blow air and being stuck by a needle. (136 S. Ct. 2160 (2016).) The Court decided the following about chemical tests that accompany proper DUI arrests:

An officer doesn't need a warrant to take a breath sample. Requiring the driver to complete a breath test constitutes a valid search incident to arrest.

It's okay to make refusing a breath test a crime. Statutes that impose criminal penalties for refusing to give a breath sample don't violate the Fourth Amendment.

An officer normally needs a warrant to take a blood sample. An officer normally can't make a driver submit to a blood draw without first getting a warrant. But the Supreme Court did indicate that, when a blood test is the only viable option—for instance, because the driver appears to be on drugs rather than drunk—the officer won't have to get a warrant if there isn't enough time to do so. And, three years after the Birchfield case, in Mitchell v. Wisconsin, the Court established that law enforcement is almost always allowed to gather a blood sample for testing without a warrant if the driver is unconscious and the police haven't been able to administer a standard evidentiary breath test. (The Mitchell case did, however, say that kind of warrantless blood draw could be unconstitutional, essentially when blood wouldn't have been drawn in the normal course of medical care and the police were pretty clearly not too busy to get a warrant. (139 S. Ct. 2525 (2019).))

It's not okay to make refusing a blood test a crime. If the Fourth Amendment typically requires warrants for blood tests, then states can't make it a crime to refuse a blood test where there is no warrant.

It's okay to impose administrative penalties for refusing a blood test. Some implied consent laws might allow license suspension where the driver refuses a blood test. They might also allow the prosecution to argue in court that the blood-test refusal shows the defendant is guilty. But as long as these laws don't make refusing a blood test a criminal offense, they're okay.

U.S. Constitution vs. State Constitutions

As noted below, the law in some states is more protective of suspects than in others. In Georgia, for example, the state constitution does not allow the government to compel someone to perform a breathalyzer test. (Even there, though, refusing a breath test can come with a price.)

A Matter of Privacy

The Supreme Court based the above rules on an evaluation of privacy interests.

Justice Samuel Alito, writing the Court's Birchfield opinion, reasoned that breath tests aren't significant privacy intrusions. He emphasized that they don't involve invasion of the body. Justice Alito equated completing a breath test with blowing up a party balloon. He also noted that the government gets nothing other than a computer readout from a breath test—there's no sample for the police to later analyze.

Blood tests, according to Alito, are far different. They involve piercing of the skin. And people don't continually shed blood in the way that they constantly emit breath. Not only that, but a blood sample that can be stored and analyzed for all kinds of information has the potential to reveal a lot more about someone than a fleeting breath sample.

Talk to a Lawyer

If you're been arrested for DUI, consult an experienced lawyer. A knowledgeable attorney can explain how the law applies to you—including whether your state's laws might offer more protections.

Talk to a DUI Defense attorney
We've helped 115 clients find attorneys today.
There was a problem with the submission. Please refresh the page and try again
Full Name is required
Email is required
Please enter a valid Email
Phone Number is required
Please enter a valid Phone Number
Zip Code is required
Please add a valid Zip Code
Please enter a valid Case Description
Description is required

How It Works

  1. Briefly tell us about your case
  2. Provide your contact information
  3. Choose attorneys to contact you