The Fourth Amendment provides that a person shall be secure in her person from unreasonable searches and seizures. United States v. Katz, 389 U.S. 347 (1967). The constitutional prohibition against unreasonable searches and seizures must liberally construed in order to safeguard this right of privacy. Boyd v. United States, 116 U.S. 616, 635 (1886) (“[C]onstitutional provisions for the security of person and property should be liberally construed. . . . It is the duty of courts to be watchful for the constitutional rights of the citizen, and against any stealthy encroachments thereon."). The Fourth Amendment's protection against unreasonable searches and seizures is so important that evidence obtained from an unreasonable search is excluded from evidence at trial. Weeks v. United States, 232 U.S. 383 (1914) (establishing exclusionary rule in federal courts for illegally obtained evidence); Mapp v. Ohio, 367 U.S. 643 (1961) (applying exclusionary rule to state court proceedings).
In Skinner v. Railway Labor Executive's Ass'n, 489 U.S. 602 (1989), a case involving federal regulations that mandated drug and alcohol testing of any railroad employee involved in a train accident, the Supreme Court addressed the issue whether a breathalyzer or urinalysis were "searches" under the Fourth Amendment. Answering in the affirmative, the Court said that although the collection and testing of urine do not entail any intrusion into the body, they nevertheless constitute searches, since they intrude upon expectations of privacy as to medical information that society has long recognized as reasonable. Id. at 616.
In Schmerber v. California, 384 U.S. 757 (1966), the Court found no violation of the Fourth amendment when a police officer took a blood sample of an unconscious driver whom the officer had reason to believe was drunk. The Court, in affirming Schmerber's conviction, held that the officer's belief that a delay in obtaining a search warrant would lead to the destruction of evidence - the dilution of the blood-alcohol ratio - was reasonable under the facts of the case. Indeed, the officer's belief was reasonable, as the liver would have synthesized the alcohol and thus "destroyed" evidence of intoxication.
In Wisconsin v. Faust, the Wisconsin Supreme Court further erorded the Warrant Clause, the details of which you can see in this post.