Do I have a choice which BAC test I take, and if so, what should I choose?
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In most states, there are three ways of determining how much alcohol you have in your system: breath or blood, and in a few states, urine. The urine test has fallen into disfavor because retention of urine for hours results in mixing in the bladder urine with higher concentrations of alcohol and urine with lower concentrations, thus distorting the results. California, for example, stopped offering a urine test in 1999, except in unusual circumstances. Usually, you can choose which sobriety test you want to take.
If you choose the breath test, you will blow into a machine called a Breathalyzer that will determine the blood alcohol in your breath. Many jurisdictions permit you to have a second test and/or a blood or urine test. This is because a breath sample is not saved and so cannot later be re-analyzed by the defense.
Breath tests are the least accurate in determining a person's blood alcohol content (BAC). This is because the breath machine assumes that the breath is from the lungs when it may be the alcohol residue still lingering from the mouth, esophagus, or the digestive system. If you belch, burp, hiccup, or vomit shortly before testing, or take a "breath freshener" (e.g., Binaca, Listerine), or cough syrups, you can bring vapor from alcoholic beverages still in the stomach up into the mouth and throat, registering an inaccurately high reading that would be used against you. Dental bridges and dental caps can also trap alcohol.
Analysis of a blood sample is potentially the most accurate, since it shows the presence of both alcohol and drugs. The least accurate and least reliable, by far, however, is urinalysis.
Thus, if you are confident that you are sober, a blood sample is the wise choice; urine, being least accurate and most easily impeached, is the best option if you believe your blood-alcohol concentration is above the legal limit and you wish to be discreet.