Why A- and B- samples?

We always use A1, A2 and B samples for extra security. The drug test is done on the A samples. If the first test and the confirmation test both turn out positive for something you know you haven’t taken, you can always request an analysis of the B sample. The B sample is for the test subject’s security. B samples are stored in a freezer according to the lab’s routines in case the A sample comes out positive.