What Is an MRO and Why Do DOT Tests Require One?

The Medical Review Officer sits between the laboratory result and the employer report. They exist specifically to ensure that a positive drug test result actually means what it appears to mean — and to protect employees from inaccurate or unfair outcomes.

What a Medical Review Officer Is

A Medical Review Officer (MRO) is a licensed physician who has received specialized training in the collection and analysis of drug specimens, the pharmacology of controlled substances, and the interpretation of drug test results. Under 49 CFR Part 40, all DOT non-negative drug test results must be reviewed by a certified MRO before the result is reported to the employer.

MROs must be certified through a nationally recognized MRO certification board and maintain their certification through ongoing continuing education. They are not employed by the laboratory — they operate independently and serve as a medical check on the testing process.

What the MRO Actually Does

When a laboratory reports a non-negative result (positive, adulterated, substituted, or invalid), they transmit the result to the MRO — not directly to the employer. The MRO then:

Reviews the laboratory documentation for procedural accuracy
Contacts the employee directly by phone to conduct an interview
Determines whether there is a legitimate medical explanation for the result
Makes a final determination of positive, negative, canceled, or refusal
Reports the verified result to the employer's Designated Employer Representative (DER)

The MRO interview is the employee's opportunity to explain the result. If the employee has a valid prescription for a substance that triggered a positive, the MRO can verify the prescription and potentially report the result as negative — if the medication's use is consistent with safe performance of the safety-sensitive function.

The MRO's Role

The employer never sees a non-negative laboratory result until the MRO has reviewed it and made a final determination. The MRO's verified result — not the raw lab report — is what drives any employment action.

What the MRO Cannot Do

MROs have clearly defined authority under Part 40, and that authority has limits. An MRO cannot change a laboratory's reported result — they can only accept it, cancel it, or verify it as negative based on a legitimate medical explanation. They cannot excuse a positive result based on secondhand smoke, passive exposure, poppy seeds (for most substances), or unverifiable claims.

An MRO also cannot discuss the result with the employer before completing their review. The employer does not have the right to receive the raw laboratory result — only the MRO's verified determination.

Why DOT Requires MRO Review

The MRO requirement exists because drug testing creates real consequences — removal from employment, loss of a CDL, mandatory SAP referral. Without an independent medical review, those consequences could be triggered by a collection error, a laboratory mistake, or a legitimate prescription medication.

The MRO functions as a factual and medical check on a process that, without oversight, could unfairly harm employees. At the same time, the MRO process strengthens the integrity of the result that the employer ultimately receives — because it has been independently verified.

MRO Review in Non-DOT Programs

Non-DOT programs are not required to use MRO review, but many employers choose to include it. An MRO review adds credibility to the result and helps defend against employee challenges. It also ensures that prescription medications are handled appropriately before any adverse employment action is taken.

Whether your program is DOT-regulated or not, MRO review is one of the strongest procedural protections available — for the employer and the employee alike.