Mach cover this one, and I have read it myself. "Anytime you are in uniform".
I have been tested 3 times in roughly 5 years. The paper you sign states that YOU will select and open the mouth piece used for testing. I have yet to be allowed to select mine. The tester has a glove on that has handled who-knows how many other samples that day.
All 3 of my test were legally invalid, and I recorded the 3rd one just incase. I use mouthwash morning and night.