The U.S. Supreme Court outlined new parameters for testing suspected drunk drivers on Thursday, ruling that the Fourth Amendment allows warrantless breath tests but forbids warrantless blood tests.

“The impact of breath tests on privacy is slight, and the need for BAC [blood alcohol content] testing is great,” Justice Samuel Alito wrote for the Court in Birchfield v. North Dakota and its consolidated cases. “We reach a different conclusion with respect to blood tests. Blood tests are significantly more intrusive, and their reasonableness must be judged in light of the availability of the less invasive alternative of a breath test.”

The Court combined three cases from lower courts in the Midwest for its ruling. All of them involved state laws criminalizing the refusal of blood and breath BAC tests. In North Dakota, Danny Birchfield received a 30-day sentence and fine for refusing a warrantless blood test and Michael Baylund lost his drivers’ license after officers told him refusing a warrantless blood test was a crime. In Minnesota, William Bernard challenged his prosecution after refusing a warrantless breath test.

Writing for the Court, Alito analyzed the tests under the Fourth Amendment’s search-incident-to-arrest exception. As its name suggests, the rule allows police officers to search a person’s body and immediate surroundings without a warrant after their lawful arrest. Breath BAC tests, he wrote, don’t rise to the level of intrusiveness the Fourth Amendment protects: they leave no permanent sample in the government’s possession and don’t undermine the arrestee’s dignity in any significant way.

But the justices in the majority refused to apply that logic to blood BAC tests, citing both the greater intrusiveness of obtaining blood and the wealth of other information that can be obtained from it.

“It is another matter, however, for a State not only to insist upon an intrusive blood test, but also to impose criminal penalties on the refusal to submit to such a test,” Alito wrote. “There must be a limit to the consequences to which motorists may be deemed to have consented by virtue of a decision to drive on public roads.”

In a separate opinion joined by Justice Ruth Bader Ginsburg, Justice Sonia Sotomayor sided with the majority on forbidding warrantless blood tests. She parted ways, however, on sanctioning warrantless breath tests, which she argued was inconsistent with both the Court’s precedents and the Constitution.

“This Court has never said that mere convenience in gathering evidence justifies an exception to the warrant requirement,” she wrote. In her conclusion, Sotomayor added, “I fear that if the Court continues down this road, the Fourth Amendment’s warrant requirement will become nothing more than a suggestion.”

Justice Clarence Thomas also wrote a partial concurrence but took the opposite approach from Sotomayor, arguing that the Fourth Amendment permitted warrantless breath and blood tests alike.

“The compromise the Court reaches today is not a good one,” he wrote in a brief three-page dissent. “By deciding that some (but not all) warrantless tests revealing the blood alcohol concentration (BAC) of an arrested driver are constitutional, the Court contorts the search-incident-to-arrest exception to the Fourth Amendment’s warrant requirement.”

Thomas instead suggested the Court should sanction the searches under the exigent-circumstances exception. That rule allows cops to forego warrants if they believe a suspect is about to destroy evidence. The human body’s ability to metabolize alcohol, he argued, produced a similar quandary for law enforcement.