In most states, you have a choice -- usually, of breath, blood or urine (some states, however, do not offer urinalysis). If you choose breath, many jurisdictions permit you to have a second test or blood or urine; this is because a breath sample is not saved and so cannot later be re-analyzed by the defense.

Analysis of a blood sample is potentially the most accurate. Breath machines are susceptible to a number of problems rendering them often unreliable. The least accurate 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.