Police do not generally need a warrant to draw the blood of an unconscious driver suspected of drunk driving, the Supreme Court ruled June 27 in Mitchell v. Wisconsin.
Gerald Mitchell was arrested for suspected drunk driving.  While on the way to the police station, he lost consciousness and had to be taken to a hospital, where police asked staff to draw his blood.  That draw showed his blood alcohol content was .222|PERCENT|.
Mitchell moved to suppress the blood-test results, but the Wisconsin courts did not.
In a four-justice plurality opinion by Justice Alito, the Court held that the exigent-circumstances exception to the Fourth Amendment’s warrant requirement will generally allow police to conduct a warrantless blood draw on unconscious drivers suspected of drunk driving.
In Missouri v. McNeely (2013), the Court held that the natural dissipation of alcohol in the blood did not justify a warrantless blood draw of a conscious driver.  In Birchfield v. North Dakota (2016), the Court held that police may conduct a warrantless breath test, but not a blood test, if there is probable cause to arrest a driver for drunk driving.
 “Today, we consider what police officers may do in a narrow but important category of cases:  those in which the driver is unconscious and therefore cannot be given a breath test,” the Court said.  “In such cases, we hold, the exigent-circumstances rule almost always permits a blood test without a warrant.”
When police encounter an unconscious, suspected drunk driver, the “case sits much higher than McNeely on the exigency spectrum,” the Court said.
 “Exigency exists when (1) BAC evidence is dissipating and (2) some other factor creates pressing health, safety, or law enforcement needs that would take priority over a warrant application,” the Court said.  “Both conditions are met when a drunk-driving suspect is unconscious.”
 “When a police officer encounters an unconscious driver, it is very likely that the driver would be taken to an emergency room and that his blood would be drawn for diagnostic purposes” anyway, the Court said.  In addition, many unconscious drivers will be found at accident scenes, where police will have many responsibilities to attend to injured people and direct traffic to prevent further accidents – all of which “may be incompatible with the procedures to obtain a warrant.”
 “When police have probable cause to believe a person has committed a drunk-driving offense and the driver’s unconsciousness or stupor requires him to be taken to the hospital or similar facility … they may almost always order a warrantless blood test,” the Court concluded.
But the Court did “not rule out the possibility that in an unusual case a defendant would be able to show that his blood would not have been drawn if police had not been seeking BAC information, and that police could not have reasonably judged that a warrant application would interfere with other pressing needs or duties.”
The Court remanded the case to allow Mitchell the opportunity to show if his case fit within that exception.
Other opinions
Justice Thomas concurred in the result, but said he would adopt a rule – rejected by McNeely — that the natural dissipation of alcohol in the blood always creates an exigency for a blood test if police have probable cause to believe a driver is drunk.
Justice Sotomayor, joined by Justices Ginsburg and Kagan, dissented.  They said the Fourth Amendment requires that “if there is time, get a warrant,” and here, there was time.
Justice Gorsuch dissented on procedural grounds.  He said the Court had originally granted cert. to decide whether Wisconsin’s “implied consent” law authorized the blood draw, but doesn’t answer that question.  He would have dismissed the case as improvidently granted.