That's only if the officer has a suspicion that they had that alcohol while they were driving. If the person is at the hospital, the officer may come to that through their later investigation, but they want to be able to do the screening-device test, for example, based on the fact that the person now has the alcohol or drug in the body and the officer also has a suspicion that they were driving.
On June 19th, 2007. See this statement in context.