If you test positive for drugs or alcohol after getting hurt at work, the insurance company does not have to pay you any worker's compensation benefits. You have to prove that you were sober at the time of your injury. You have to prove that you had, in legal terms, the normal use of your mental and physical faculties at the time that you got hurt.
One way you do it is by getting witness statements. You'll have to get coworkers to write statements or they will agree to appear for you at a hearing and testify you were sober at the time you were injured. You'll have to have some type of a drug or alcohol expert testify as whether or not that was a legitimate drug test and whether or not the numbers truly reflected whether or not you were impaired at the time of the injury.
Those types of cases are cases that are highly specialized. You do not want to handle one of those types of cases alone. You need to talk to a lawyer immediately if you get a letter from the insurance company saying they've denied your claim because you tested positive for any type of illegal substance.