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The MRO's Role in DOT Drug Testing

Every DOT drug test result goes through a Medical Review Officer (MRO) before it reaches you. The MRO is a doctor who checks the lab result and the paperwork, and talks to the driver when a result is not negative. This guide covers what the MRO does under 49 CFR Part 40, Subpart G, and what it means for your company.

Updated September 28, 2026 · Checked against the federal regulations

Who can be an MRO

An MRO must be a licensed physician, either a Doctor of Medicine or a Doctor of Osteopathy (40.121(a)). The MRO must finish qualification training and pass an exam from a nationally recognized MRO certification board. Requalification training and a new exam are required every five years (40.121(c) and (d)).

Under 40.123, the MRO acts as an independent and impartial gatekeeper for the accuracy of the testing process. The MRO decides whether there is a legitimate medical explanation for a positive, adulterated, substituted, or invalid lab result.

The verification interview

For negative results, the MRO checks the custody and control form (CCF) and the lab report before releasing the result (40.127).

For a non-negative result, the MRO must talk directly with the driver, in person or by phone (40.129(a)(4)). The MRO's staff can set up the call but cannot gather medical information (40.131(b)).

• The MRO or staff try at least three times over 24 hours to reach the driver (40.131(c)).
• If that fails, the MRO asks the DER to tell the driver to call the MRO. The MRO does not tell the DER why (40.131(c)(2)).
• The MRO can verify the result without an interview if the driver declines to talk, if 72 hours pass after the DER reached the driver, or if no one can reach the driver within ten days (40.133(a)).
• Unless the employer has a stand-down waiver under 40.21, the MRO cannot tell the employer about a lab-confirmed positive before it is verified (40.129(g)).

Prescriptions and other explanations

The driver has the burden to show a legitimate medical explanation (40.137(c)). A legally valid prescription can be one. The MRO does not question whether the doctor should have prescribed it, and the MRO can check with the pharmacy (40.137(a) and 40.141). The MRO can give the driver up to five days to produce records.

If there is a legitimate medical explanation, the MRO verifies the test as negative (40.137(d)). Some explanations are never accepted (40.151):

• A doctor's recommendation for a Schedule I drug, such as under a state medical marijuana law.
• Hemp or other non-prescription marijuana-related products, or coca tea.
• Stories of passive or unknowing use, like a spiked drink or a marijuana brownie.
• Poppy seeds for morphine or codeine at or above the high cutoff levels (40.139(b)).

Safety concerns after a negative

A verified negative does not always end the matter. Before collecting medical information, the MRO must warn the driver that some information can be shared with third parties without consent. That includes the employer, if a medication or condition could affect safety-sensitive work (40.135(d)).

For a legally prescribed medication, the MRO allows 5 business days from reporting the verified negative for the prescribing doctor to call. The goal is to see if the drug can be changed to one that is not a safety risk (40.135(e)). If the safety risk remains, the MRO may raise it with the employer.

Dilute, invalid, and split specimens

Dilute: the MRO reports the dilute finding with the negative or positive result (40.155). A positive-dilute is treated as a verified positive. For a negative-dilute with creatinine from 2 to 5 mg/dL, the MRO will tell you to recollect right away under direct observation. For other negative-dilutes, you may retest, but you must treat all employees the same (40.197).

Invalid: if the driver has no acceptable medical explanation, the MRO cancels the test and tells you to recollect right away under direct observation (40.159(a)(5)).

Split specimen: after a verified positive, adulterated, or substituted result, the driver has 72 hours to ask the MRO to test the split specimen at a second lab (40.153 and 40.171). The employer must make sure the test happens. The driver cannot be required to pay before the test, but the employer may seek reimbursement.

How and when you get results

The MRO reports results to your DER. If you choose, results can come through your C/TPA instead (40.165). Either way, the timing rules in 40.167 apply:

• Verified positives, refusals, adulterated or substituted results, and results that need an immediate observed recollection must go to the DER the same day the MRO verifies them or the next business day. A phone call is preferred.
• The written MRO report must reach the DER within two days of verification.
• The MRO does not give quantitative drug levels to the DER or C/TPA (40.163(g)).
• The MRO reports verified positives and MRO refusal determinations to the Clearinghouse within 2 business days (382.705(a)).
• Pact Compliance keeps MRO reports organized in your drug and alcohol records. Your current lab and MRO stay in place.

Common questions

Sources

• 49 CFR 40.121 (eCFR)
• 49 CFR 40.123 (eCFR)
• 49 CFR 40.127 (eCFR)
• 49 CFR 40.129 (eCFR)
• 49 CFR 40.131 (eCFR)
• 49 CFR 40.133 (eCFR)
• 49 CFR 40.135 (eCFR)
• 49 CFR 40.137 (eCFR)
• 49 CFR 40.139 (eCFR)
• 49 CFR 40.141 (eCFR)
• 49 CFR 40.151 (eCFR)
• 49 CFR 40.153 (eCFR)
• 49 CFR 40.155 (eCFR)
• 49 CFR 40.159 (eCFR)
• 49 CFR 40.163 (eCFR)
• 49 CFR 40.165 (eCFR)
• 49 CFR 40.167 (eCFR)
• 49 CFR 40.171 (eCFR)
• 49 CFR 40.197 (eCFR)
• 49 CFR 382.705 (eCFR)