On your first offense DUI, with a CE charge doented as a misdameanor, the jail takes your saliva?

when you stay the night instead of bailing out - If you plead not guilty, situation: X calls to say you smell of alchohol, you drive home with your children, span of 25minutes, not impaired while driving, have drink at home, they come in on investigation allegation and give you tests, arrest you blow .12 . Why do they need DNA/Saliva?