The chief concern in this situation is to strictly adhere to your institutions policy and procedure, whatever that may be. Different health care institutions have varying policies on matters such as this. A second point is to not judge the person based on the test results of the urine specimen. All individuals that work in health care must learn to be tolerant of others and keep their own opinions to themselves. Basically, what you think about the person or the urine results has no bearing on the care that you must provide them. You must treat every patient with the same amount of courtesy and respect. Thirdly, if in fact the person consumed the illicit substance, what physical or psychological manifestations might occur?
Further, was the chain of custody observed in regards to the urine sample? If not, the test results could be false. Also, until we examine the maintenance records of the specific instrumentation that ran the test, a one hundred percent positive test result can't be ascertained. Machines, just like people, make errors.