In  Missouri v. McNeely, a majority of U.S. Supreme Court justices said that the Constitution does not authorize police to obtain a blood specimen from someone who they suspect is DUI without ever having to comply with the Fourth Amendment’s a warrant requirement.  There is no hard and fast rule for all cases where the police want to take someone’s blood for further analysis in suspected DUI cases. A majority of the justices agreed that whether a warrant is required will be determined on a case by case analysis.