This isn't true. I just got out of the drug testing industry and my mother also does drug testing at the hospital she works at and I can tell you that the described story is not how they do things. They must follow strict guidelines.

If they think you adultered the test, then they would tell you that and fail you. If they believe you substituted (ie. they find a bottle on you or the temp is not right), they make you retest immediately while someone is in the room with you. A test that is "inconclusive" or a "No result" is considered an automatic retest due to lab error.

Either this poster is a liar for some reason or the clinic did not follow protocol.