SCOTUS ping.
Generally, a search requires a search warrant, but there is an exception for "exigent circumstances." Back in 1966, in a case called Schmerber v. California, SCOTUS ruled that drawing blood from a suspected drunk driver was always an "exigent circumstance," because the blood alcohol would dissipate before a warrant could be obtained.
Yesterday's decision held that Schmerber is no longer valid, for two reasons: (1) today, police can get a warrant much more quickly than they could in 1966, because they can fax or email a request to a judge who can fax or email the warrant back; and (2) today's blood alcohol tests are more sensitive and need not be done as quickly to be valid.
The majority did not hold that a warrant is always necessary, but said that, any time the police draw blood without a warrant, they will have to show that the facts of the case prevented them from doing so.
The concurrence (Roberts, Alito and Breyer) generally agreed, but wanted the Court to give more guidance to the police about what facts would justify not getting a warrant. Only Thomas wanted to uphold the Schmerber rule.
This will have interesting repercussions in states which are decriminalizing pot, as breathalyzers are useless.