On April 17, 2013, the U.S. Supreme Court decided Missouri v. McNeely, No. 11-1425, holding that the natural metabolization of alcohol in the bloodstream does not present a per se exigency categorically justifying a warrantless, nonconsensual blood test in a drunk-driving case. Exceptions for exigency in this context require a case-by-case analysis.

The defendant was arrested after a Missouri police officer pulled him over on suspicion of drunk driving and he refused to take a blood-alcohol breath test at the scene. The officer then transported the defendant to a nearby hospital for a blood test, and he again refused his consent. The officer did not seek a warrant but instead instructed a lab technician to obtain a blood sample, which demonstrated that the defendant's blood-alcohol content exceeded the legal limit. During his subsequent prosecution for drunk driving, the defendant moved to suppress the blood-test results, arguing that the warrantless, nonconsensual blood test violated the Fourth Amendment. Both the trial court and the Missouri Supreme Court agreed. The latter held that Supreme Court precedent "requires more than the mere dissipation of blood-alcohol evidence to support a warrantless blood draw in an alcohol-related case."

The Supreme Court affirmed. The Court noted that its precedent demands a case-by-case analysis when lower courts determine whether exigent circumstances justified a warrantless search. Although the State argued that exigency necessarily exists in any alcohol-related blood test given that blood-alcohol content rapidly diminishes with time, the Court found no reason to adopt a per se rule. The Court agreed that significantly delaying a blood test to obtain a warrant would "negatively affect the probative value of the results." But it reasoned that when the state has time to obtain a warrant, the Fourth Amendment requires it to do so. When obtaining a warrant is impractical, the blood testing may well merit an exigency exception. Because the State based its argument solely on the proposed per se rule, the Court declined to detail the relevant factors courts must weigh when analyzing exigency in drunk-driving cases.

Justice Sotomayor delivered the opinion of the Court, in which Justices Scalia, Kennedy, Ginsburg and Kagan joined. Justice Sotomayor also delivered an opinion as to Part III (concerning other arguments for a per se exigency rule), in which Justices Scalia, Ginsburg and Kagan joined. Justice Kennedy filed an opinion concurring in part. Chief Justice Roberts filed an opinion concurring in part and dissenting in part, in which Justices Breyer and Alito joined. Justice Thomas filed a dissenting opinion.

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