Add something like this from UPS's contract?
"Note: (Reasonable Cause) At the time the urine specimen is collected, the employee may opt to also give a blood sample. If the employee takes this option, the blood sample must confirm positive presence for the substance confirmed in the urine test. If no positive is confirmed in the blood specimen, the employee will be given a warning letter, offered an opportunity for rehabilitation as set forth in this Article, and the employee will be required to otherwise satisfy the requirements imposed by the DOT regulations. However, if there is a second occasion where reasonable cause testing results in a positive urine test, the employee will then be subject to discharge."