Ok, here's the thing. This device WILL NOT be able to tell EXACTLY when you last used a drug... Everyone is different; e.g., one's thc level after smoking will be different than another person's level. This is one of the many, many reasons that marijuana is still illegal, because there's no 100% sure test to determine if one is high AT THIS VERY SECOND. Hard to develop such a test for a drug that can have a long half-life and whose byproducts become stored in fat.
Because of that I don't think if one was actually high and got tested but refused to admit it to the officer and went to court they could be charged with a DUI / OUI DRUGS. Would a prosecutor be able to argue such a case and prove guilt beyond a reasonable doubt?