Blood alcohol concentration is the percentage of alcohol that is in the blood at the time of testing. This is the key. In many states, your blood alcohol levels must be at a certain level at the time you were driving. There are several types of tests: breath analysis, urinalysis and blood analysis. The most accurate test is the blood analysis, yet it can have many problems with accuracy. Due to the variance between people's metabolisms, one level may mean a different level of impairment than another. The prosecution realizes these variances and generally wants a blood alcohol level significantly above the legal level of intoxication. If the accused did not appear to be intoxicated, the accuracy of the test may be challenged. There are many other technical reasons for challenging the blood alcohol test. In other words, it is not written in stone. The information contained in this message is general and should not substitute for the advice and counsel of a licensed attorney.