I doubt if a DCS drug screen could result in an arrest.
However, it MIGHT result in a deeper law enforcement criminal investigation and subsequent charge.
Drug abusers aren't arrested for consuming that poison.
Drug abusers are usually charged for possessing or selling the poison.
No. An arrest would not happen.
DCS could use the test result to take other action within iit jurisdiction, but it would not result in a new criminal charge.
Not only is this not anything to worry about, you likely don't have to take the tests in the first place.
Unless the court has ordered you to test there is nothing wrong with politely declining. If DCS wants you to test they can obtain a court order.