In some states, employers must use a certified lab. These states include Connecticut, Hawaii, Idaho, Kansas, Louisiana, Maryland, Minnesota, Mississippi, Montana, Nebraska, North Carolina, Oklahoma, Oregon, Utah, Vermont, and West Virginia.
Outside of these states, there is no requirement to use a lab, but using an outside lab gives you the advantage of using a neutral third party and trained professionals. This may be helpful if a positive drug test result is challenged. All employers who test in-house may want to use a lab to confirm a positive test.