(Carolyn Kaster/AP)

Police officers generally must try to get a warrant before forcing uncooperative drunken-driving suspects to submit to a blood test, the Supreme Court ruled Wednesday.

The natural dissipation of alcohol in a person’s bloodstream does not justify an exception to the general constitutional requirements of a warrant, Justice Sonia Sotomayor wrote for the majority.

She said such emergencies must be determined by the circumstances in a case-by-case examination and rejected the notion that officers face a “now or never” situation in obtaining blood alcohol tests.

“In those drunk-driving investigations where police officers can reasonably obtain a warrant before a blood sample can be drawn without significantly undermining the efficacy of the search, the Fourth Amendment mandates that they do so,” Sotomayor wrote.

She was joined in her main holding by Justices Antonin Scalia, Anthony M. Kennedy, Ruth Bader Ginsburg and Elena Kagan.

Chief Justice John G. Roberts Jr. agreed with the outcome of the case, which affirmed a decision from the Missouri Supreme Court, but criticized the vagueness of the majority’s test.

“A police officer reading this court’s opinion would have no idea — no idea — what the Fourth Amendment requires of him, once he decides to obtain a blood sample from a drunk driving suspect who has refused a breathalyzer test,” wrote Roberts, who was joined by Justices Stephen G. Breyer and Samuel A. Alito Jr.

Strong words aside, Roberts’s proposed rule differed mostly by degree.

“If there is time to secure a warrant before blood can be drawn, the police must seek one,” Roberts wrote. “If an officer could reasonably conclude that there is not sufficient time to seek and receive a warrant, or he applies for one but does not receive a response before blood can be drawn, a warrantless blood draw may ensue.”

Justice Clarence Thomas was the lone justice agreeing with Missouri and the U.S. government that the metabolization of alcohol in the blood created the kind of emergency that does not require a warrant.

The case came from rural Cape Girardeau County, where in the early-morning hours of Oct. 3, 2010, Missouri State Highway Patrol Cpl. Mark Winder pulled over Tyler G. McNeely. McNeely, whose speech was slurred and who had alcohol on his breath, failed a field sobriety test and twice refused to take a breath test.

Winder arrested him and, on the way to jail, stopped by a hospital. After McNeely refused to submit to a blood test, Winder ordered a phlebotomist to draw blood anyway. Winder did not attempt to obtain a warrant because he said he thought Missouri law did not require it because of recent changes.

The Missouri Supreme Court unanimously disagreed and said the blood test could not be used. Courts nationwide were divided on the issue.

Sotomayor acknowledged that “cases will arise when anticipated delays in obtaining a warrant will justify a blood test without judicial authorization.”

But she disputed the notion that evidence would always be lost because of the time it takes to obtain a warrant. Some delay is inevitable anyway, she wrote, because the officer must transport a suspect to a medical facility to have the test conducted.

Officers can use that time to get a warrant, she said. “Well over a majority of states allow police officers or prosecutors to apply for search warrants remotely through various means, including telephonic or radio communication, electronic communication such as e-mail, and video conferencing,” she wrote.

She said the court recognizes the problem of drunken driving, but that about half of the states require warrants for blood draws and they have not experienced unreasonable difficulty. Maryland, Virginia and the District of Columbia do not have a warrant requirement.

Sotomayor said the decision also complies with “our recognition that any compelled intrusion into the human body implicates significant, constitutionally protected privacy interests.”

The case is Missouri v. Mc­Neely.

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