I know of someone who used synthetic urine for a job application and got found out. When asked about it by the interviewer he said he had a medical card but was still afraid of a positive test. The interviewer/recruiter was very sympathetic but couldn't take him on as the employer's policy...
In that case, I would think you would have to make sure the person is the same sex as you and if you are a woman, that her pregnancy status is the same as yours!