Do I have legal grounds to sue this agency?

I live in a rural area and all the nearby towns within reasonable travel distance are fairly small. Almost all of the major employers(manufacturing) in my area use the same local staffing agency to hire employees. I went to this agency and filled out paperwork, interviewed, tested, and took a drug screen. About two weeks had gone by and they called with a job opening. I went in to fill out more paperwork and was told it had been more than 7 days since my last drug screen and I would have to test again. This time they said I failed and that my test was positive for THC. This was an error because I haven't done drugs of any kind. I begged them to retest me and they refused. They said it was their policy to only do one test and because I failed the test that I would be ineligible to be employed with them for a period of 4 years. The only way to rectify the situation was to immediately go to industrial health and pay $70 for my own test that same day to prove that I was clean. I was currently out of work so I didn't have the money to do this. Now because I can't work for them I can't get a job at A LOT of places where I live. Every place I find that pays anything substantial uses this agency. I swear the cup/test was defective and now I have been cheated out of any chance at a decent job. This has seriously had a devastating impact on my life. Now I'm forced to search for work outside of my field of experience and having a terrible time finding work. Can I sue them for this?