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I've wondered this for years. Finally a case has ended up in the Supreme Court and we will find out.
From the NYT's coverage:
Prosecutors in Missouri, supported by the federal government, came to the Supreme Court on Wednesday with a big request: They wanted the justices to rule that the police do not need warrants to obtain blood samples in drunken-driving investigations.
There seemed little enthusiasm among the justices for that categorical approach. Instead, the argument turned into a search for a middle ground that would take account of the practical realities of roadside stops, body chemistry and the administration of justice in the digital age.
On the one hand, the natural dissipation of blood alcohol means that time is of the essence when people suspected of drunken driving are pulled over and refuse to consent to a breath test. Obtaining a warrant, moreover, takes time.
On the other hand, several justices expressed discomfort with what Chief Justice John G. Roberts Jr. called the “pretty scary image” of government-sanctioned bodily intrusions involving sharp needles.
