I think we have to get this clarified.
I agree with you that the testing of bodily fluids is at the end of the process. This is after the person was stopped. The officer has reason to suspect you have drugs in your body. You fail the standard field sobriety test. They have reasonable grounds to believe you are impaired by drugs. They take you to the station. They do the eleven-part test of DRE. They conclude you have a particular drug.
At the end of that, you're right, they get to demand and do an invasive test. It's just like how, if I'm charged with an indictable offence, the police can fingerprint me. I may find that to be invasive. If the police charge me with particular offences, they can demand hair, saliva, DNA. So you're right, it is, but it's at the end of a long process in which there are a number of safeguards and barriers built in. I think we have to recognize that. If, at any one of those stages, the officer concludes that there are no drugs and you're not impaired, you don't pass go and you don't go to the next step.
The other thing that I think it's important to emphasize is the other aspects of the DRE that establish the impairment. I would take issue with the view that DRE testing can't establish that you're impaired. That's absolutely not so, because it can establish that you are impaired. As a matter of fact, the science on divided attention, past testing—horizontal gaze and various test elements are clearly linked to impairment.
So I think you're right, it is invasive, but it's at the end of a long number of barriers that you have to jump over, all of which you have to answer yes to. Every other element of the DRE goes to impairment. The other elements go to the impairment by drug, including some of the physical examination stuff, but you're right, the bodily fluid test only confirms presence of drug.