A positive drug test result rarely goes unquestioned. An employee disputes it, a union representative asks for the paperwork, or an arbitrator wants to know exactly how the result was produced before accepting it as grounds for termination. When that happens, the question an employer actually has to answer is not “was the person using drugs.” It is “can we show, step by step, that this specific result is accurate and was produced the right way.” That second question is what chain of events and documentation are built to answer, and it is where a lot of otherwise solid testing programs run into trouble.
Here is what actually makes a result hold up when someone pushes back on it.
The result has to come from a certified lab, not just any lab
For federally regulated testing, and as good practice even outside DOT rules, the specimen has to be analyzed by a laboratory certified under the Department of Health and Human Services’ National Laboratory Certification Program. SAMHSA maintains the current list of certified laboratories, and it is worth checking that whichever lab a testing program uses actually appears on it, rather than assuming based on a vendor’s own marketing. A lab’s certification can lapse or be suspended, and a result from a lab that is not currently certified is a weak result no matter how clean the rest of the process looks.
This matters because certification is what forces a lab to run specific, validated methods rather than whatever process is cheapest. A certified lab has to follow documented quality control procedures, pass regular proficiency testing, and use confirmatory testing methods that can stand up to scrutiny. None of that is visible to the employer day to day, but it is the foundation everything else sits on.
A positive screen gets confirmed, not just reported
An initial screening test, whether it is an immunoassay run on urine or a similar screen on an oral fluid sample, is designed to be sensitive. That sensitivity means it can react to substances that are chemically similar to the drug it is looking for, which is why a non-negative screen is never the final word. A legitimate program sends any non-negative screen to a second, more specific method, typically gas chromatography-mass spectrometry or a comparable confirmatory technique, before it is reported as positive.
If a result an employer is relying on was never confirmed, that alone can sink it the moment it is challenged. Confirmation is not an optional extra step. It is the difference between a screening result and a verified result, and only the second one should ever be the basis for an employment decision.
A trained medical professional reviews the result before anyone acts on it
Under the DOT framework, a confirmed positive, refusal, or other non-negative lab result does not go straight to the employer. It goes to a Medical Review Officer first. The MRO’s job is to review the laboratory result, give the employee a confidential opportunity to explain it (a legitimate prescription, for example), and only then verify it as positive, negative, or in some other category, before reporting a verified result to the employer. The Department of Transportation lays out exactly what an MRO is required to do with a confirmed result under 49 CFR 40.135, and that review step is a large part of why DOT-regulated results tend to hold up better under challenge than results from programs with no equivalent review in place.
Even outside DOT-covered testing, building something like this into a non-DOT program, a qualified reviewer who checks for legitimate medical explanations before a result reaches HR, closes off one of the most common ways a result gets successfully challenged: “I had a valid prescription and nobody asked.”
The paperwork matches the specimen, every step of the way
This is the part that trips up more programs than any actual lab error does. A result holds up when the specimen’s custody and control documentation is complete and internally consistent from the moment of collection through the moment the lab reports a result: correct identifiers, matching specimen numbers, proper seals, signatures at each handoff, and a clear record of who had the specimen at every point. A single mismatched ID number or a missing signature is exactly the kind of gap a challenge will focus on, because it is concrete and easy to point to, unlike arguing about lab science.
Programs that treat this paperwork as a formality to rush through at collection time are the ones that end up unable to answer “how do we know this result belongs to this employee” when someone actually asks.
The policy was applied the same way it always is
A technically perfect lab result can still get undermined if the process around it looks inconsistent. If an employer tests some employees under a reasonable suspicion policy without documented, specific observations, skips steps for people managers like, or applies different consequences to similar results, a challenge stops being about the chemistry and becomes about fairness and consistency instead. That is a much harder argument for an employer to win, and it has nothing to do with whether the drug test itself was accurate.
Holding every test, every specimen type, and every employee to the same documented process is what keeps a program defensible as a whole, not just defensible test by test.
A common misconception: a lab error is the usual challenge
Employers sometimes brace for a dispute assuming the employee’s side will attack the chemistry, arguing the lab made a mistake. In practice, that is rarely where a successful challenge comes from, because certified lab error rates are low and confirmatory testing is specifically designed to catch screening mistakes. The far more common and more successful challenges target the process around the test: a broken chain of custody, a missed MRO review, inconsistent policy application, or a collection that did not follow the documented procedure. Programs that pour all their attention into “is the lab reliable” while treating documentation as paperwork are protecting against the wrong risk.
What this means for choosing a collection method
The specimen collection step is where chain of custody either starts clean or starts with a problem. A collection method that is quick, observed, and simple to document consistently reduces the number of places where something can go wrong between collection and the lab. This is part of why many employers running on-site collection, for pre-employment, random, or post-incident testing, have moved toward oral fluid collection using a mouth swab drug screen: the collection itself is observed directly, there is no bathroom privacy step, and the specimen handling from collection to sealed packaging is straightforward to document correctly every time. None of that replaces proper chain of custody paperwork, but it does remove some of the common points where that paperwork gets fumbled.
Building a result that can survive being questioned
None of this requires a legal department. It requires a program where the lab is actually certified and checked against the current list, non-negative screens are always confirmed before anyone acts on them, a qualified reviewer looks at results before they reach a manager, the custody paperwork is treated as part of the test rather than an afterthought, and the policy gets applied the same way every time. Put those five things in place consistently, and a disputed result becomes something the employer can walk through step by step, rather than something they have to hope nobody looks at too closely.
Sources
- SAMHSA: Drug-Free Workplace Certified Laboratories List, https://www.samhsa.gov/substance-use/drug-free-workplace/drug-testing-resources/lab-list
- U.S. Department of Transportation, Office of Drug and Alcohol Policy and Compliance: 49 CFR 40.135, What are the MRO’s functions in reviewing laboratory confirmed drug test results, https://www.transportation.gov/odapc/part40/40-135
Written and reviewed by Sarah, MouthSwabDrugTests.com
