You may sign papers consenting to being drug tested, however, they cannot just take a sample from you without telling you AT THAT TIME, that they are drug testing you. It's called informed consent and a person has the right to withdraw consent at anytime. That is why, if you've ever, ever been drug tested, you'd know it. They cannot collect a sample from you without informing you that what you had consented to by signing that paper in the beginning is being carried out.
Also, due to the cheap cost of urine testing vs. blood testing for drugs, almost universally a urine sample will be taken for an initial drug screen. If it's positive, a blood sample will usually be drawn to confirm and have a more accurate reading. Urine can be concentrated or watered down due to natural processes.
Every maternity appointment I had, I was told to pee in a little cup with no lid, no label and no one watching. They could have tested it for drugs and it would never have held up in court if I were positive. No one saw me pee in the cup, there is no label to distinguish it from the other 10 samples on the counter and it was not collected using semi sterile protocol.
So, one could sign all they want without reading, but you would still know if a sample was being obtained for a drug screen.
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