I had a drug test come back positive (swab) for heroine. can a hair follicle test prove that i have been clean of any substances?

Not necessarily: The only SPECIFIC test for heroin is a test for its metabolite 6-monoacetylmorphine or 6MAM. I am not sure whether this is available in swab form, but the first thing with ANY positive test on a screening test, such as a urine drug screen or oral fluid swab is to do the confirmatory test. If they have not done that, they are not following procedure. There also needs to be chain-of-custody on it.