DUI Blood testing for blood alcohol level is the most accurate form of chemical test that exist. However, it can still be riddled with problems. One of the benefits of blood testing is that the sample is retained and can be re-tested by the accused driver. A disadvantage however is that a driver can be tested for substances other than alcohol that may be used against him in the prosecution of a DUI charge. Such substances include cocaine, methamphetamine, marijuana, vicodin, valium and a myriad of other drugs both illegal and prescribed. A breath test can only measure alcohol. A blood test taken outside of 3 hours from the point of driving prevents DMV from suspending the license. In the criminal prosecution it goes to the weight of the evidence and a good defense attorney can show that the measurement is unreliable because of the delay. Often times a police agency does not have a blood technician on hand and must wait for one in order to draw blood. The more delay the better (most of the time). As a driver, you have a right to request a blood test rather than a breath test.