Each case is different.  I look at DUI defense in 3 steps.  First, did the police have enough reason to stop you.  They have to see something to give them “reasonable suspicion” that you violated a law.  Second, did they have enough to arrest you.  They have to have “probable cause” to believe you are DUI.  Third, is the blood or breath test (sometime urine test) valid.  That is, can they prove in court that you had enough alcohol or drugs in your system to get a conviction.