The US Supreme Court takes up a case on Wednesday that examines whether police must obtain a warrant from a neutral judge before forcibly extracting blood from a suspected drunk driver.

At issue is when police have the authority to force a motorist to submit to a blood test after the motorist refuses a breathalyzer test.

Under the Fourth Amendment, Americans are protected from unreasonable searches and seizures. That means before police invade a person’s privacy, they must demonstrate to a neutral judge that there is probable cause to believe a crime has been committed. If the judge is convinced, a search warrant is issued.

That’s what happens in most cases. But the Supreme Court has ruled that under certain circumstances police are free to conduct a search without first obtaining a warrant.

The question in Wednesday’s case, Missouri v. McNeely (11-1425), is whether a state trooper violated motorist Tyler McNeely’s Fourth Amendment rights when the trooper ordered a hospital attendant to forcibly take a blood sample from a hand-cuffed Mr. McNeely to preserve evidence of his elevated blood-alcohol level.

The argument for allowing warrantless blood tests is centered on the fact that blood alcohol levels fall with time.

The Supreme Court is being asked to resolve a split among state supreme courts on the issue. The top courts in Wisconsin, Minnesota, and Oregon have all ruled that the rapid dissipation of alcohol in the bloodstream is a sufficient exigency to justify a warrantless blood test.

In contrast, state high courts in Iowa, Utah, and Missouri have ruled that dissipation does not excuse police from obtaining a warrant before conducting an involuntary blood test.

The Obama administration and attorneys general from 32 states, the District of Columbia, and Guam are urging the Supreme Court to declare that warrantless blood tests do not violate the Fourth Amendment.

In the case before the court Wednesday, McNeely was pulled over in October 2010 by Missouri State Highway Patrol Cpl. Mark Winder. The trooper says he saw McNeely’s pickup truck speeding and veering across the highway’s centerline.

After stopping McNeely, the trooper noticed the driver had bloodshot eyes, slurred speech, and a strong odor of alcohol on his breath. McNeely was given a field sobriety test, including the one-leg stand test. He performed poorly.

That’s when Corporal Winder asked McNeely to submit to a machine breath test. McNeely refused.

Winder placed McNeely under arrest for driving while intoxicated. On the way to the county jail, the trooper stopped at the hospital. After McNeely refused to submit to a blood test, the officer directed an attendant to forcibly extract a sample of McNeely’s blood.

The test revealed that McNeely had a blood alcohol content of 0.154 percent, significantly above the legal limit of .08 percent.

McNeely was charged with driving while intoxicated. He had two prior convictions for drunk driving and faced up to four years in prison, if convicted.

McNeely’s lawyer filed a pre-trial motion seeking to exclude the blood sample as evidence because, the lawyer argued, it was obtained without a judicially-authorized warrant and thus violated McNeely’s right to be free from unreasonable searches.

The trial court agreed and excluded the evidence. A state appeals court reversed. The case went to the Missouri Supreme Court, which agreed with the trial court that the involuntary taking of blood from a suspect without first obtaining a warrant violated the suspect’s Fourth Amendment rights.

In their appeal to the US Supreme Court, prosecutors from Cape Girardeau County, Missouri, argue that the trooper was justified in taking the forced blood sample without first obtaining a warrant because any delay in attempting to contact a prosecutor and judge late at night would facilitate the continuing destruction of evidence.

Alcohol in the bloodstream of a drunk driver is certain to dissipate through normal metabolism, Assistant Prosecuting Attorney John Koester said in his brief.

“The decision of the Missouri Supreme Court … actually requires the police to stand by and allow the best and most probative evidence of the crime to be destroyed during a drunk driving investigation,” Mr. Koester wrote.

“Such an approach is wholly inconsistent with core principles of the Fourth Amendment,” he said.

The Supreme Court has permitted law enforcement officials to conduct warrantless searches when necessary to prevent the imminent destruction of evidence. Koester says that’s what happens with each passing minute during a drunk driving investigation.

“Under these circumstances, it is reasonable for an officer to direct medical personnel at a hospital to draw a blood sample from a drunk driver without first obtaining a warrant,” Koester said.

“A prompt blood test, taken with as little delay as possible, provides the best and most probative evidence of intoxication,” he said. “The privacy interests of the individual, on the other hand, are minimal.”

Lawyers for McNeely disagree. They argue that there is no reason the trooper couldn’t have quickly obtained a judicially-authorized warrant before forcibly extracting McNeely’s blood.

Prosecutors and judges are routinely available around-the-clock, they say.

“This was not an emergency, it was a run of the mill driving while intoxicated case,” Steven Shapiro, a lawyer with the American Civil Liberties Union, which is defending McNeely, wrote in his brief.

“As in all cases involving intoxication, the defendant’s blood alcohol was being metabolized by his liver. However, a prosecutor was readily available to apply for a search warrant and a judge was readily available to issue a warrant,” Mr. Shapiro said.

Shapiro said that a 1966 Supreme Court decision authorizes warrantless blood-alcohol tests, but only when there are exigent circumstances that might delay the testing long enough that the evidence will be destroyed.

In McNeely’s case there was no accident to be investigated and no one was injured requiring emergency medical treatment at the side of the road. Shapiro says under those circumstances there is no exigency that might justify an officer failing to obtain the required warrant.

Koester replies that a blood test is a “minor intrusion” that cannot counterbalance the public’s interest in highway safety.

“The public interest in ridding the nation’s roadways of drunk drivers clearly outweighs the privacy interests of the individual in being subjected to a simple blood test,” Koester said.