If you want to dispute a positive drug test result, you have a narrow window to act and a specific sequence to follow: speak with the Medical Review Officer when they call, request a retest of your split specimen within 72 hours, and gather every piece of documentation that could explain the result. Under federal Department of Transportation rules, that 72-hour clock starts the moment the MRO notifies you of the verified positive.1U.S. Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.171 Miss it, and your strongest built-in appeal disappears.
What follows is the sequence that gives you the best chance of overturning the result, in the order the deadlines force you to take them.
The Two Deadlines That Matter Most
Two 72-hour windows control almost everything about a DOT-regulated dispute.
The first runs from the moment your employer’s Designated Employer Representative tells you to contact the MRO. If you don’t call within 72 hours, the MRO can verify the result as positive without ever hearing your explanation.2GovInfo. 49 CFR Part 40 Section 40.135
The second runs from the moment the MRO tells you the result has been verified positive. You have 72 hours to request that your split specimen be sent to a second laboratory for independent testing.1U.S. Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.171
Private employers who aren’t subject to DOT rules set their own timelines, and those can be even tighter. Pull out your employee handbook or drug testing policy the same day you’re notified and find every deadline written into it.
Talk to the MRO Before Anything Else
In DOT-regulated and federal workplace testing, the Medical Review Officer is a licensed physician who reviews confirmed positive lab results before they become “verified” positive results reported to your employer.3eCFR. 49 CFR Part 40 Subpart G – Medical Review Officers and the Verification Process A lab-confirmed positive is not the final word. The MRO has to talk to you first, and the conversation is your first real chance to make the result go away.
When the MRO calls, they must tell you which drug was detected, walk you through the verification interview, and explain that their decision turns on the information you give them.2GovInfo. 49 CFR Part 40 Section 40.135 If you have a valid prescription for a medication that could cause the positive, the MRO must verify the test as negative.3eCFR. 49 CFR Part 40 Subpart G – Medical Review Officers and the Verification Process
Do not skip or delay this interview. Come to it prepared. Have your prescription bottles or pharmacy printout in front of you. Know the name, dose, and prescribing doctor for every medication. If a medical condition could explain the result, have your records ready to describe. The MRO uses medical judgment to decide whether your explanation is legitimate, so specificity matters. “I take something for my blood pressure” is weak; the drug name, prescriber, and pharmacy is strong.
Request a Split Specimen Retest
When a DOT-regulated urine sample is collected, the collector splits it into two bottles. Bottle A gets tested first. Bottle B, the split specimen, sits sealed at the lab. If the MRO verifies your result as positive and you still believe it’s wrong, you can ask for Bottle B to be sent to a second HHS-certified lab for independent analysis. The request can be verbal or written, and again, you have 72 hours from MRO notification to make it.1U.S. Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.171
Once you ask, the MRO must immediately direct the first laboratory to send the split to the second lab. If the second lab fails to confirm the original finding, the MRO must cancel the test.1U.S. Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.171
Cost is not a barrier at the front end. Under DOT rules, your employer is responsible for ensuring the split specimen gets tested in a timely manner regardless of whether you can pay for it. The employer may later seek reimbursement through company policy or a collective bargaining agreement, but they cannot condition the retest on your ability to pay upfront.4eCFR. 49 CFR 40.173 – Who Is Responsible for Paying for the Test of a Split Specimen
Private employers not subject to DOT regulations aren’t bound by these split specimen rules. Many follow similar protocols voluntarily because it reduces liability, but there’s no federal requirement that a private employer offer MRO review or a split retest. Your rights in that situation depend on your employer’s written policy and applicable state law, so the employee handbook is again the starting document.
Identify Why the Result Might Be Wrong
The MRO call and any retest are only as good as the explanation you can offer. Most false positives fall into a handful of categories, and identifying which one fits your situation tells you what evidence to gather.
Medications and Supplements
Dozens of ordinary medications can trigger a positive on an immunoassay screening test. Over-the-counter decongestants containing pseudoephedrine or phenylephrine can flag as amphetamines. Ibuprofen and naproxen have been linked to false amphetamine results. Bupropion, sertraline, trazodone, and venlafaxine (all antidepressants) are known culprits. Diphenhydramine, the active ingredient in many sleep aids and allergy medicines, can cross-react on screening panels. Certain antibiotics, proton pump inhibitors, and HIV medications have triggered false results for benzodiazepines and other controlled substances. Dietary supplements are another risk area, particularly weight-loss products containing DMAA.
If you took any prescription, over-the-counter drug, or supplement in the days before your test, that information becomes the centerpiece of your dispute.
Food and Hemp Products
Poppy seeds remain a real source of false opiate results. They contain trace morphine and codeine, and a large serving before a test can push urine concentrations above detection thresholds. The federal workplace program raised its initial opiate cutoff to 2,000 ng/mL specifically to reduce poppy-seed false positives, but results near that threshold are still worth disputing. Hemp-derived CBD products, which legally may contain up to 0.3% THC, can accumulate enough THC metabolites with regular use to trigger a marijuana positive on screening.
Medical Conditions
Diabetes, particularly when poorly controlled, can produce ketones that interfere with some assays. Liver and kidney disease affect how quickly your body clears substances, which can elevate metabolite levels above cutoff thresholds for reasons unrelated to drug use.
Hair Test Contamination
If your test was a hair follicle test, external contamination is a legitimate ground for dispute. Being in a room where marijuana or crack cocaine is smoked can deposit drug residue on your hair from the outside. Research has documented that THC can appear in hair from secondhand marijuana smoke exposure, and that cocaine incorporation into hair involves complex mechanisms beyond simple ingestion.5SAMHSA. Hair External Contamination: Literature Review
Collection and Lab Errors
Procedural mistakes at collection or in the lab can invalidate a result. Chain of custody gaps in the paperwork tracking your sample are among the strongest grounds for challenge. Sample mislabeling, improper storage temperatures, calibration errors, and unqualified technicians can all undermine reliability. If you noticed anything unusual during your collection (a broken seal, a skipped temperature check, an interrupted procedure, missing signatures on the custody form), write it down the same day. Notes made immediately after the event carry far more weight than a recollection reconstructed weeks later.
Document Everything
Whether you’re preparing for the MRO call, a split retest, or a formal appeal, your dispute lives or dies on documentation. Gather the following as early as you can:
- A pharmacy printout showing every medication dispensed in the 30 days before the test, including prescriber, dosage, and fill date. A letter from your doctor explaining how a specific medication could produce the flagged positive carries significant weight with an MRO.
- A list of every over-the-counter product and supplement you took, with brand names and active ingredients.
- Medical records establishing any condition (diabetes, kidney disease, liver disease) that could explain the result, along with your treatment status at the time of testing.
- A food log if poppy seeds or hemp-derived CBD are in the picture, with quantities and timing relative to the test.
- Written notes on any collection irregularity, made as close to the event as possible.
- A running log of every call, email, and meeting related to the result, with dates, times, names, and what was said.
Get the Lab’s Litigation Package
If the dispute is heading toward a formal challenge or legal action, request a litigation package from the laboratory that analyzed your specimen. This is the lab’s complete technical file on your test, and it gives an attorney or toxicologist the raw material to attack the scientific validity of the result.
A standard package from an HHS-certified laboratory includes a description of the testing procedures performed, copies of the federal chain of custody form and all internal custody documents, detailed descriptions of the screening and confirmatory testing methods and instrumentation, the actual test data for your specimen along with all calibrators and quality controls, and the résumés of the certifying scientist and other personnel who handled the analysis.6SAMHSA. Medical Review Officer Manual – Chapter 6 Additional Medical Review Officer Responsibilities
This is where lab errors become visible. A toxicologist reviewing your package can spot calibration problems, quality control failures, or chain of custody gaps that aren’t apparent from the positive-or-negative result the employer sees. Request it early to give your team time to work through it before any hearing.
Filing a Formal Dispute With Your Employer
If the MRO interview and split retest don’t resolve the matter, a formal internal dispute is the next step. For DOT-regulated employees, the MRO’s verification decision is the key gatekeeping step, but your employer may still have an internal appeal process. For non-regulated private employers, HR or a third-party administrator typically runs the process, and the steps should be spelled out in the drug testing policy.
Your written submission should identify the specific grounds for the dispute, attach every supporting document organized so a reviewer can follow it, and state the relief you’re asking for: overturning the result, ordering a retest, reinstating your position, or all three. Send it by a method that creates proof of delivery, and keep copies of everything.
Watch the deadlines closely. Many employer policies and collective bargaining agreements set tight windows after notification, and missing one can forfeit your right to challenge regardless of the merits.
If the Internal Dispute Is Denied
A denial isn’t necessarily the end. Several paths may remain depending on your situation.
If your employer has a multi-level grievance process, use every step. Arbitration under a collective bargaining agreement has overturned positive results where employers failed to follow their own testing policies or where chain of custody problems emerged.
An employment attorney can evaluate whether you have grounds for legal action. Possible claims include wrongful termination if the employer didn’t follow its own policy or applicable state law, disability discrimination under the Americans with Disabilities Act if a legitimate prescription triggered the positive, or defamation if the result was disclosed improperly. Under the ADA, if the employer takes adverse action based on a positive test and you have a protected disability, the employer must show you cannot perform essential job functions or that you pose a direct threat that can’t be reduced through reasonable accommodation.7U.S. Equal Employment Opportunity Commission. Enforcement Guidance on Disability-Related Inquiries and Medical Examinations of Employees under the ADA That doesn’t protect illegal drug use, but it can protect employees whose positive traces to a legitimately prescribed medication for a disability.
Keep documenting through every appeal. The record you started building on day one is what turns a he-said-she-said dispute into a case with evidence behind it.
Special Situations Where the Rules Change
A few contexts don’t follow the standard workplace playbook, and you need to know if you’re in one of them.
CDL holders and other DOT-regulated drivers. A verified positive is reported to the FMCSA Drug and Alcohol Clearinghouse, and the driver is immediately prohibited from performing any safety-sensitive functions for any DOT-regulated employer. The return-to-duty process requires a DOT-qualified Substance Abuse Professional, whatever education or treatment the SAP recommends, a negative return-to-duty test, and at least six unannounced follow-up tests in the first twelve months back on the job.8Federal Motor Carrier Safety Administration. Drug and Alcohol Clearinghouse Return-to-Duty Mailer As of November 2024, a “prohibited” status in the Clearinghouse can also cost you your state-issued commercial driving privileges. This is why the split specimen stage matters so much for drivers: undoing a Clearinghouse entry is far harder than preventing verification in the first place.
Marijuana. Cannabis remains a Schedule I controlled substance under federal law, and all DOT-regulated testing treats a THC positive as a violation with no exceptions for state legalization. No MRO can verify a marijuana positive as negative based on a state medical marijuana card or the fact that recreational use is legal where you live. Outside DOT, roughly nine states as of 2025 prohibit employers from firing or refusing to hire someone solely because of legal, off-duty marijuana activity, provided the employee isn’t impaired at work. These protections almost universally exclude safety-sensitive positions, federal contractors, and DOT-regulated workplaces. If you’re in a state with off-duty protections and not in one of those excluded roles, state law may give you grounds for a challenge even when the test itself is scientifically valid.
Probation and custody cases. Court-ordered testing often lacks the procedural safeguards built into DOT testing. There may be no MRO review, no guaranteed split, and no formal dispute process. If you believe a court-ordered test produced a false positive, ask immediately whether the lab ran a GC-MS or LC-MS-MS confirmation on the positive screen. If only a screening immunoassay was run, push for confirmatory testing, and consider getting an independent test at a certified lab the same day. Bring documentation of any medications to your probation officer and, through your attorney, raise the issue with the court before a revocation hearing. In child custody disputes the same principles apply: request confirmatory testing, document medications, and retain an attorney who can challenge testing methodology.
The single thread running through every version of this is speed. Every hour you spend deciding whether to fight is an hour off one of the clocks that determine what you can still do. Make the MRO call. Ask for the split. Start the paperwork.