This is a very good question. There are two kinds of chemical tests under California state law to determine a driver’s blood alcohol content (BAC) at the time he was stopped on suspicion of driving under the influence of alcohol, drugs, or both. Until recently, there was also a urine test which was part of the drivers choice of tests. This test after many years. was eliminated by the legislature due to their determination that this type of test did not meet the high standards of accuracy required by the law.
There are two remaining tests which a suspected drunk driver has to chose from. These tests are the blood and breath tests. The blood test, which most experts view as the more accurate of the two because this test essentially through a direct analysis, measures the percentage of alcohol in the blood drawn from a suspected driver.The result is a blood alcohol concentration or bac which is directly correlated to the language of the law prohibiting blood-alcohol levels of .08 or over.
In addition, if the suspected driver is also under the influence of a drug, a blood test or urine test is the only way to determine their presence. A breath test is only capable of measuring alcohol.