I Have An Employee Who Took A DOT Random Drug

I have an employee who took a DOT random drug test but was unable to produce 45ml of urine within a 3-hour period. She was allowed to drink up to 40 ounces of liquids as prescribed in DOT regulations CFR 40.193 (b) (2). The Medical Review Officer for the Company's drug testing program has instructed me to send her for a "shy bladder" evaluation. What is involved in such an evaluation?