In all states, an adult who drives with a blood alcohol content (BAC) level of .08% or above is guilty of a DUI or DWI. For drivers under the age of 21, almost all states set the limit at .01% or .02%. Police officers often administer chemical tests to measure the BAC in your breath, blood, urine, or saliva. The results of these tests are usually a big part of the prosecution's DUI case against you. If you can successfully challenge the validity of these tests, the test results may be deemed inadmissible at trial. (Read more about blood alcohol content and how it is measured.)
In many states, if you refuse to take a chemical test, your license may be automatically suspended. If arresting officers fail to advise you of this consequence, the results of the test might not be admitted in court or in a license suspension hearing. Similarly, some states require police officers to offer a choice between tests. If officers fail to comply with the law, this could affect the admissibility of test results.
States usually have requirements as to how chemical tests must be administered and how the machines are calibrated and maintained. If you can prove that the officers did not comply with these requirements, that the machine was faulty, or that the technician was not competent, you may be able to get the test results thrown out.
A wide range of factors can affect chemical tests, rendering the test results inaccurate. Challenging the accuracy of chemical BAC tests often requires technical and legal expertise in the subject area or the testimony of a forensic chemist or other expert. Examples of conditions that might change the results of the test include:
Most breath testing machines will register any number of chemical compounds in human breath as alcohol, including some types of food and non-impairing drugs or medications. Eating food or taking drugs within certain periods prior to the test may result in a false reading.
It takes about 45 minutes to three hours for alcohol to become fully absorbed into your body. This means that if you "have one for the road," you may not be legally impaired while driving, even if a chemical test administered later shows a BAC over the legal limit (your BAC will rise between the time you were driving and the time that the test was administered -- often an hour or more after the police stop you).
Mounting many of these DUI defenses may require the expertise of a lawyer. If you are accused of a felony DUI or DWI, you should hire an attorney -- be sure to find one experienced with handling DUIs or DWIs. Those facing lesser charges -- misdemeanors or license suspensions -- may also benefit from consulting with a lawyer. To learn more about finding a defense lawyer, see Nolo's article Criminal Defense Lawyer FAQ.