Blood tests are taken by people who are pulled over by law enforcement and are suspected of having consumed alcohol above the limit allowed for driving. They can also be taken to expose drug use. Different types of drunk driving can be misdemeanors and some can be felonies; the determination of which is largely dependent on the blood test results.
There are two reasons why a blood test would be required for a person that was pulled over for suspected driving under the influence. The first reason is if the person refuses to perform a breathalyzer; in this instance, the police have the authority to detain the suspect and take them to a hospital where a mandatory blood test is taken. The second reason is if the person complies with the breathalyzer request, but cannot perform the test accurately. This can happen due to severe asthma and cannot breathe as hard as is necessary to complete the test.