Supreme court requires warrant for blood alcohol test

In the 2013 case  Missouri v. McNeely, the U.S. Supreme Court held an involuntary blood draw is a "search" under the Fourth Amendment. Therefore, a search warrant is generally required, subject to certain exceptions. The court further held that in a routine DUI case such as this, there is no general emergency exception to a search warrant. 

The argument for an emergency exception made is that a suspect's blood alcohol content naturally dissipates, therefore time is of the essence and a warrant cannot be reasonably obtained without unduly undermining police investigation. 

The Court reasoned that technology now allows a warrant to be obtained extremely quickly via telephone or email, and that magistrates are typically  available 24 hours a day to provide such warrants. Any emergency exception to the general warrant requirement are always to be decided on a case-by-case basis, but the facts of this case do not constitute an emergency.

You can find the Supreme Court opinion here: http://www.supremecourt.gov/opinions/12pdf/11-1425_cb8e.pdf.