Im no legal eagle but from what I picked up if you have blood and cuts in your mouth (chomp), have recently consumed alcohol or have another reason for having a compromised oral membrane and/or a gag reflex issue that will prevent you scraping your tounge without vomiting you will more than likely get taken back for the drager test or a blood test. (But that is far more likely to come up positive).
But, if they DO continue with saliva testing you may have a defense that the test results were not reliable.
If you do get the full saliva testing make sure you get a second test done for independent testing!!!!
Your demeanor, the cop and your luck on the day play a huge role in how it plays out and they are experts at tricking you into thinking you have to answer and manipulating you.
But basically, no comment.. no comment.. no comment. to everything.
You are a good citizen who does not use illegal substances and anything that is there is without your knowledge or consent.
If you wish to plead not guilty I'd suggest that you deal with it via a submission or pretrial mediation because depending on your state/territory trying it before a mag will be FAR less likely to go in your favour.
Take all of that with a grain of salt and Im not telling snyone to break the law