The Supreme Court ruled Wednesday that police usually must try to obtain a search warrant from a judge before ordering blood tests for drunken-driving suspects.
The justices sided with a Missouri man who was subjected to a blood test without a warrant and found to have nearly twice the legal limit of alcohol in his blood.
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Justice Sonia Sotomayor wrote for the court that the natural dissipation of alcohol in the blood is generally not sufficient reason to jettison the requirement that police get a judge's approval before drawing a blood sample.
Missouri and the Obama administration were asking the court to endorse a blanket rule that would have allowed the tests without a warrant.
Eight of the nine justices rejected that plea. Only Justice Clarence Thomas would have held that a warrantless blood test does not violate a suspect's constitutional rights.
The case stemmed from the arrest of Tyler McNeely in Missouri's rural Cape Girardeau County. A state trooper stopped McNeely's speeding, swerving car. The driver, who had two previous drunken-driving convictions, refused to submit to a breath test to measure the alcohol level in his body.
He failed several field sobriety tests. The arresting officer, Cpl. Mark Winder of the Missouri State Highway Patrol, said McNeely's speech was slurred and he was unsteady on his feet.
There seemed little dispute that Winder had enough evidence to get a warrant for a blood test, but chose not to. Instead, he drove McNeely to a hospital. A technician drew blood from McNeely, who was handcuffed throughout the process.
McNeely's blood-alcohol content was 0.154 percent, well above the 0.08 percent legal limit.
But the Missouri Supreme Court upheld a lower court order that threw out the results of the blood test. The state high court said the blood test violated the Constitution's prohibition against unreasonable searches and seizures. Police need a warrant to take a suspect's blood except when a delay could threaten a life or destroy potential evidence, the Missouri court said.
About half the states already prohibit warrantless blood tests in all or most suspected drunken-driving cases.
The Supreme Court did not offer much guidance Wednesday about when police may dispense with a warrant. Justice Anthony Kennedy, in a separate opinion, said a later case may give the court to opportunity to say more on that subject.
The case is Missouri v. McNeely, 11-1425.