It is Federal Regulation that we must maintain a “bluing” agent in the tank and bowl of any commode used for urine drug testing. The reason is so that donors cannot attempt to substitute water from the commode as their urine specimen or use that water to “dilute” their urine.
It is much harder for a lab to detect drugs in a specimen that has been diluted by either adding water to the specimen or if the donor consumes large amounts of water/liquid.