There are three kinds of blood alcohol content tests that can be performed. If you are showing signs of intoxication, you may be asked to take one.
The first kind is a breath test. This is the most common method used today. Breathalyzers work by measuring how much alcohol passes through the lungs and is expelled into the air. The breathalyzer estimates the level of intoxication, but it is prone to errors. For instance, if you burp, more alcohol could come out in your breath, ruining that particular test. Environmental factors can also influence the test, rendering it null and void.
A blood test is another way to check the alcohol levels in your body. Typically, you have to give consent for this test. A blood test is usually more accurate than other kinds of alcohol-discovering tests, but it can still be flawed. Vial mix-ups, fermentation of blood in the vial and coagulation can all affect the test.
A urine test is the last kind of test that can be issued. These are the least accurate and are only used when they’re the only option. Urine tests are typically much higher or much lower than the actual BAC of the body. This is because the urine is a reflection of the time passed since that particular drink passed through the body; alcohol takes up to two hours to appear in urine and may stay in the person’s system for up to 24 hours.
Each of these tests can be flawed, which is why it’s a good idea to question their validity in your defense.
Source: BAC Track, “Three Types of BAC Testing,” accessed Sep. 28, 2016