28 CFR 550.31: Procedures.

(a) Staff of the same sex as the inmate tested shall directly supervise the giving of the urine sample. If an inmate is unwilling to provide a urine sample within two hours of a request for it, staff ordinarily shall file an incident report. No waiting period or extra time need be allowed for an inmate who directly and specifically refuses to provide a urine sample. To eliminate the possibility of diluted or adulterated samples, staff shall keep the inmate under direct visual supervision during this two-hour period, or until a complete sample is furnished. To assist the inmate in giving the sample, staff shall offer the inmate eight ounces of water at the beginning of the two-hour time period. An inmate is presumed to be unwilling if the inmate fails to provide a urine sample within the allotted time period. An inmate may rebut this presumption during the disciplinary process.

(b) Institution staff shall determine whether a justifiable reason exists, (e.g., use of prescribed medication) for any positive urine test result. If the inmate's urine test shows a positive test result for the presence of drugs which cannot be justified, staff shall file an incident report.