Following a person’s arrest for a DUI/DWI criminal offense, police officers will ask the accused/defendant to submit to a blood test or blood draw in which a qualified person such as a nurse at the police station or hospital will draw the defendant’s blood (usually from the arm) into a vial to be tested for alcohol concentration and ultimately to determine whether the person is per se intoxicated. Per se intoxication is intoxication by definition—as defined in the state’s statutes, which is often a blood alcohol concentration (BAC) of .08 or higher.

All states have implied consent laws that make a driver’s consent or agreement to submit to a blood, breath, or urine test (a chemical test) when requested by a law enforcement officer a condition of the driver’s acceptance of a driver’s license offered by the state. Despite implied consent laws, in 2013 the United States Supreme Court held that if a person refuses to submit to having their blood drawn the police cannot draw the blood by force without a search warrant, as required under the Fourth Amendment to the United States Constitution. But more recently the Supreme Court upheld a nonconsensual blood draw of an unconscious person suspected of driving under the influence—based on the state’s implied consent laws and the exigent circumstances exception to the Fourth Amendment's warrant requirement.

And if a driver refuses to take a blood or breath test there are other consequences even if the police do not secure a search warrant for the blood draw—including admission of the refusal in evidence at trial, fines, penalties, and automatic suspension of the driver’s license for a year or more.

In Pennsylvania, the law regarding DUI/DWI offenses includes the concept of 'per se intoxication,' where a person is considered legally intoxicated if their blood alcohol concentration (BAC) is .08 or higher. Under Pennsylvania's implied consent law, drivers have implicitly agreed to submit to chemical tests (blood, breath, or urine) for the purpose of determining BAC when lawfully requested by a police officer. Refusal to submit to such tests can result in various penalties, including the introduction of the refusal as evidence at trial, fines, and an automatic suspension of the driver's license, typically for a period of one year. Following the 2013 U.S. Supreme Court decision, police officers in Pennsylvania cannot forcibly draw blood without a search warrant, except in certain circumstances such as when the driver is unconscious and exigent circumstances are present. This is in line with the Fourth Amendment's protection against unreasonable searches and seizures. However, even without a warrant or a blood sample, the consequences of refusing a chemical test can be significant.