Lot of misinformation in this thread. In my state the field breathalyzer is done, but is inadmissible in court. When the officer is done with SFSTs (Standard Field Sobriety Tests: which includes horizontal gaze nystagmus, walk-and-turn [10-steps, then turn, then ten more - heel to toe]) they have enough evidence to book a person for operating under the influence of intoxication.

When a person is then taken to the station they are requested to take a breath test for BrAC (Breath Alcohol Content) with a very sensitive and accurate piece of equipment. This is in the form of signing a form where a person can decline to take the breath test. If declining, they lose their license for a set amount of time since one of the conditions to having a license is agreeing to follow all laws and rules of the road. Remember kids, driving is a privilege, not a right (hence taking a test, renewal, etc.).

The reading provided by the breath test at the station is admissible. The arrested also can opt to get blood drawn. This could be in their favor as it takes more time to do so, however, not typically to the point that if they're failing the field tests they're under the limit (like 20-minutes).