Norfolk Southern Pre - Placement Exam

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I know that there are a lot of posts of people saying they do not use drugs when they fail the hair screening but in my case I have never touched illegal drugs. I had my hair test with Norfolk Southern which came back positive for Cocaine. I have not had any medical procedures in the last 2 years so that rules out any false positives. I knew that my being positive was impossible so I paid out of pocket for a hair test at a DOT certified clinic. That test came back as expected negative. Norfolk Southern Health services says they do not retest and that the results of their test are final. What recourse do I have? Obviously a lab error has occurred and now I am disqualified from the position I was applying for and future opportunities with Norfolk Southern.