Sigh......Let me walk you through a DUI arrest.

Stop vehicle for infraction, one that is usually a sign of impairment. Not maintaining it's lane, wide turn, etc. Reason for stop is justified. Make contact with driver, driver appears to be impaired, Note that manual dexterity is poor, speech is slurred, I now have reason to believe that this person is impaired. Person is removed from the car, given Standard Field Sobriety Tests or SFSTs for short. (horizontal gaze nystagmus, walk and turn). Subject fails SFSTs. You then read Implied consent to the subject. (You have operated a vehicle in X state, and in X state when operating a vehicle you must consent to a chemical test because you have exercised the privilege of operating a vehicle on x state's roadways. If you do not consent to a chemical test, a warrant for your blood will be issued within x hours (usually only 1 or 2) and your license will be automatically suspended for X amount of days) If the subject refuses to blow on the actual breathalizer at the station or jail or whereever, you advise subject they are now under arrest, you then get the warrant signed by the judge, transport the prisoner to the local hospital have a nurse draw blood for testing, then transport the prisoner back to the Jail, this person is arrested for "Refusal". More charges will come later as the BAC results return.
If the subject consents to a breathalizer (a legal large machine at the station/jail/whereever) once they blow, and it is above .08 then the subject is placed under arrest at this time. They are charged with actual DUI.
Everything that occurried during the traffic stop was nothing more than steps leading up to the actual arrest. Admitting to drinking even 100 beers or failing a portable breath test during the traffic stop itself is not legal grounds for an arrest.