A Medical Review Officer (MRO) is a person who is a licensed physician and who is responsible for receiving and reviewing laboratory results generated by an employer's drug testing program and evaluating medical explanations for certain drug test results.  The MRO or Medical Review Officer helps protect both the rights of the employee being tested and the employer requiring the testing.  The MRO may also assist institutions or organizations to develop and implement drug testing policies as well as to implement clinical recommendations for positive results.
The Medical Review Officer acts as an independent and impartial "gatekeeper" and advocate for the accuracy and integrity of the drug testing process. The MRO provide quality assurance review of the drug testing process, determine if there is a legitimate medical explanation for laboratory confirmed positive, adulterated, substituted and invalid drug test results, ensure the timely flow of test result and other information to employers and protect the confidentiality of the drug testing information.
The MRO may work as an independent contractor or an employee for the Department of Transportation, independent laboratories, hospitals, schools, etc.
The following are some of the MRO’s specific responsibilities as required by the Department of Health and Human Services (DHHS) SAMHSA Guidelines:
Receive all drug test results from a NIDA and/or AHCA certified laboratory.  Request, if needed a quantitative description of test results, when allowable.   Receive and review copy of the original “Chain of Custody” document to determine if there are any fatal or correctable errors that may require cancellation of a test.   Review and interpret confirmed positive drug test results.  Conduct a medical interview with the donor for any confirmed positive, adulterated, substituted, or invalid test results.  Review the donor’s relevant medical history and obtain any necessary medical or prescription records.  Consult with laboratory officials as necessary.   Notify donor who has a verified positive result that he/she has seventy two hours in which to request a test of the split specimen at another SAMHSA certified laboratory.  
The MRO shall at all times comply with requirements specified in the DOT Procedures for Transportation and Workplace Drug and Alcohol Testing Programs, 49 CFR Part 40 as well as other state statutes.      
The MRO must  also have knowledge about   the pharmacology of drugs of abuse; accepted pharmacological treatment and standard prescribing practices for specific disease process; use and authorization to prescribe controlled substances consistent with Drug Enforcement Agency (DEA) rules and regulations; ethical considerations in workplace drug testing programs; laws and regulations related to the use of illicit and licit substances; chemical dependence and addiction behavior; and employee assistance programs and rehabilitation.