In what circumstances will the police be able to obtain a blood sample from a suspect without cause of arrest?

In United States v. Dionisio, --- U.S. ---, 93 S.Ct. 764, 769, 35 L.Ed.2d 67, 76, the court said: 'As the court made clear in Schmerber, supra, the obtaining of physical evidence from a person involves a potential Fourth Amendment violation at two different levels--the 'seizure' of the 'person' necessary to bring him into contact with government agents (citation) and the subsequent search for and seizure of the evidence. In Schmerber, we found the initial seizure of the accused justified as a lawful arrest, and the subsequent seizure of the blood sample from his body reasonable in light of the exigent circumstances.'