A Medical Review Officer (MRO) is a licensed physician with knowledge of substance abuse disorders and DOT drug testing regulations. Under 49 CFR Part 40, the MRO verifies test results, contacts the donor to discuss positive results (the "verification" interview), evaluates legitimate medical explanations (such as valid prescriptions), and makes the final determination to report a result as positive, negative, or cancelled. The MRO serves as a gatekeeper to protect the accuracy and fairness of the drug testing process.