At the police station, the arrest will be offered a chemical test of breath, blood or, much less frequently, urine. Breath test results are usually available immediately; urine and blood samples are sent to a lab for later analysis to determine the BAC or possible presence of drugs.
If the arrest refuses to submit to chemical testing, they will usually be booked for driving under the influence; there will be no evidence for filing the second charge of driving with .08% blood alcohol content. In some cases the arrest may be charged with DUI even after passing a breathalyzer test if he or she refuses also to take subsequent urine or blood tests. However, the refusal will carry increased penalties on the driving under the influence charge (typically a longer license suspension and/or an increased jail sentence), and the act of refusing may be admissible in court as evidence of "consciousness of guilt". In some states, refusal to submit to a chemical test can result in an automatic suspension of driving privileges, regardless of whether the suspect is convicted of DUI.
In an increasing number of jurisdictions, if the suspect refuses to take a chemical test the police in some states may restrain the individual and forcefully withdraw blood. This is particularly common in situations involving an accident with injury or death. In some jurisdictions this may require obtaining a warrant from a judge. Some states have 'implied consent' laws, which you agree to when you apply for a driver's license. By signing the driver's license application, you are implying your consent to allow a law enforcement officer draw blood for the purpose of determining your BAC, should you be arrested on suspicion of DUI.
Even if you refuse a breath test, because of the 'implied consent', officers can forcibly take a blood sample for BAC determination. Some commentators, such as Brown University's Jacob Appel, have criticized the role of medical personnel in this process. According to Appel, "If physicians acquiesce today in the removal of a resistant patient's blood, soon they may be called upon to pump the contents of an unwilling patient's stomach or even to perform involuntary surgery to retrieve an evidential bullet."
While chemical tests are used to determine the driver's BAC, they do not determine the driver's level of impairment. However, state laws usually provide for a rebuttable legal presumption of intoxication at blood alcohol levels of .08 or higher.
Breath and urine tests can only estimate the BAC at the time the test is taken, which can be different than when the vehicle was actually operated.
source:
http://en.wikipedia.org/wiki/Drunk_driving_in_the_United_States#Chemical_test
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