Warrantless blood draws in Missouri are impermissible absent "exigent circumstances." The mere fact that blood alcohol dissipates over time is not, standing alone, sufficient to allow law enforcement to forego the requirement of obtaining a warrant before the blood draw.
The Fourth Amendment to the U.S. Constitution protects persons from unreasonable search, which warrantless search is presumed to be in the absence of exigency. "The evanescence of blood-alcohol was never special enough to create an exigent circumstance by itself." Other special facts that risk disappearance of evidence, like delays in obtaining judicial authorization caused by an accident investigation, are necessary to constitute an exigent situation.
If law enforcement obtains a blood draw over the objection of the person suspected of intoxicated driving, and does so without obtaining a search warrant from a judge beforehand, then the results of that blood draw, including any other incriminating evidence obtained as a result of the warrantless search, are suppressed. In other words, the jury never hears about the test at trial. If the test results are central to the prosecution's case, then the prosecution might decide to dismiss the charge if the test results are suppressed. However, the prosecution may try to prove the case at trial with other, more circumstantial evidence.
State of Missouri, Appellant, v. Tyler G. McNeely, Respondent.
Missouri Supreme Court - SC91850