In most states, if someone is arrested for DUI, DWI, OUI, OWI, drunk driving, or a related charge, they are required to provide a sample of their blood or breath for testing for alcohol content. (Where someone is suspected of driving under the influence of drugs, they must give a sample of their blood or urine.) The DMV typically advises people that as a condition of getting their driver's license, they have consented to a chemical test if they are suspected of DUI. This is called the "implied consent" law, and most states have some version of it.

If someone accused of drunk driving or DUI refuses to take a chemical test, there are several possible consequences: First, this refusal may be charged as a separate crime; next, the refusal can be used to suspend or revoke the person's driving privileges in a separate DMV action; finally, the fact of the refusal can be offered against the person at trial to show "consciousness of guilt."

If someone arrested for DUI or DWI has refused the test, it does prevent the prosecution from introducing evidence of blood alcohol level. However, the collateral consequences are potentially severe. Unfortunately, in most states, a person arrested for driving under the influence does not have the right to consult with an attorney before deciding whether to give a sample of their blood or breath, or deciding what type of sample to give.

There is no simple answer about which type of test to take. Breath test machines do not retain a portion of the breath to be independently retested. However, there are many ways to impeach the accuracy of the machines. Many people are not suitable candidates for breath testing, even if the machines are working properly. This can be the result of dental problems or persistent heartburn problems, both of which invite mouth-alcohol error by the machines.

Blood testing is generally thought to be the most accurate type of testing, but it too is subject to many challenges and problems. Contamination from an alcohol swab used to sterilize the area of the blood draw can compromise the blood sample. Failure to properly mix the preservative and anti-coagulant in the blood sample can cause the sample to ferment and literally create its own alcohol. Also, there is only one blood sample drawn (as compared with the usual two breath samples), so if there is a problem with that one sample, there are significant problems with the prosecution's case.

Urine samples, because of the wide array of problems and challenges, are generally thought of as the least reliable of the three types of chemical tests available. However, they are, in most states, only given as an option where a person is arrested for driving under the influence of drugs, not driving under the influence of alcohol.

 A common problem in DUI or DWI cases is where an officer fails to tell someone about their rights to have a choice of chemical tests. In most states, a DUI or DWI suspect has the option of giving a sample of their blood or breath. If more than drunk driving is suspected, namely driving under the influence of drugs, then the DUI or DWI suspect may choose a urine test. In most states, if the officer fails to give a drunk driving suspect their choice of tests, and insists on one particular test, that test result can be suppressed.

In most states, if an officer fails to tell a DUI suspect about the consequences of refusing the chemical test, and the person does refuse, the fact of that refusal may not be allowed into evidence. Usually, where someone arrested for DUI or DWI refuses to take a blood or breath test, that refusal can be introduced against them as consciousness of guilt. The prosecution's theory is that only a guilty person would refuse to take the test. However, if someone is charged with drunk driving, and they aren't properly told of the consequences of refusal, namely that their driver's license will be suspended or revoked, the refusal may not be used against them.

In most states, where someone is arrested for DUI (driving under the influence) or DWI (driving while intoxicated or driving while impaired), they are accused of failing to operate their car in the same careful manner as a sober person would in those same circumstances.

However, most states, due to pressure from the federal government relating to highway funds, have also enacted "per se" laws. Those laws establish a legal limit, usually .08%. If someone is caught with .08% or higher, as established by a test of their blood, breath, or urine, they can be charged with DUI or DWI. Even if the person doesn't exhibit any of the typical signs of drunk driving, and is able to drive just fine, they are charged with DUI or DWI based only on their body chemistry.

The prosecutor will usually charge both crimes: the traditional DUI or DWI, where they accuse someone of being unable to drive with the same caution as a sober person, and the "per se" violation of being above the legal limit. A conviction on one charge is the same as a conviction on the other. They are two ways of charging the same crime. It just gives the prosecutor two bites at the same apple. If someone is convicted of both drunk driving and being above the legal limit, they are only punished once.