By HealthDataConsortium.org Research Team
A urine sample comes back positive for THC. To most readers, that single word — positive — sounds like a verdict: this person used marijuana recently, and they are or were impaired. Neither half of that assumption is something the test itself can support. A drug test result is a measurement against a fixed administrative threshold called a cutoff concentration, and understanding what that threshold does and does not represent is the difference between reading a lab report correctly and misreading it.
What a Cutoff Concentration Actually Is
A cutoff is a specific concentration of a drug or its metabolite, set in advance, that a specimen must reach or exceed before a laboratory reports the result as positive. It is not a biological line between “affected” and “not affected.” It is an administrative and analytical line: below it, a lab calls the result negative regardless of what is actually present in trace amounts; at or above it, the lab calls the result positive regardless of when the substance was used or how the person is currently functioning.
For federal workplace and transportation-industry testing, these thresholds are not arbitrary or set test-by-test. They are published in regulation. Under the U.S. Department of Transportation's drug-testing rule, 49 CFR § 40.85, the cutoff concentration for the initial immunoassay screen for marijuana metabolites is 50 nanograms per milliliter (ng/mL), and the cutoff for the confirmatory test — gas chromatography/mass spectrometry or liquid chromatography-tandem mass spectrometry — is 15 ng/mL. A specimen must clear both thresholds, using two different testing methods, before it is reported as a confirmed positive.
The Substance Abuse and Mental Health Services Administration (SAMHSA), through its Division of Workplace Programs, sets parallel Mandatory Guidelines for Federal Workplace Drug Testing Programs that apply the same two-tier structure — an initial screen followed by a confirmatory test at a lower, more specific cutoff — for testing across the federal workforce. The consistency between the DOT and SAMHSA frameworks is a useful anchor point: these are not casual industry rules of thumb, they are numbers written into federal regulation, updated through formal rulemaking, and applied uniformly across regulated employers.
Why the Line Is Administrative, Not a Measure of Impairment
This is the distinction that gets lost in most consumer-facing explanations of drug testing: a cutoff tells a laboratory when to stop calling a result negative. It was never designed, and is not used, to tell anyone when a person was last impaired.
The American College of Medical Toxicology addressed this directly in a formal position statement on interpreting urine THC metabolite results, stating plainly that “a positive assay result does not necessarily indicate recent use” and that “neither the HHS testing nor clinical urine drug testing can be used in isolation to establish impairment or intoxication.” The active compound responsible for the subjective and functional effects of cannabis, delta-9-tetrahydrocannabinol (THC), has a duration of action measured in hours. The metabolite that urine tests actually screen for, THC-COOH, is not the active compound — it is a downstream, inactive breakdown product, and its presence in urine reflects that the body processed THC at some point, not that the person is currently under its effects.
Why the Metabolite Outlasts the High by So Much
The gap between “detectable” and “impaired” exists because of basic pharmacology. THC and its metabolites are lipophilic, meaning fat-soluble, and according to Mayo Clinic Laboratories, this property combined with a long elimination half-life means THC and THC-COOH “can be detected in urine for weeks to months after the last usage.” The compound is stored in fatty tissue and released back into the bloodstream gradually over time, long after any subjective effect has ended.
How long a person tests positive after their last use varies enormously and depends on use pattern, not on how “high” they were. The American College of Medical Toxicology's position statement notes that in chronic, heavy users, THC-COOH “may be detectable for a month or longer” after last use, driven by factors including use history and individual body composition (adiposity). An occasional user clears the metabolite far faster than a daily user, even though both may have felt identical acute effects at the time of use. That means two positive results at the same cutoff concentration can represent two completely different timelines of use — which is precisely why the number on the report cannot be read as a clock.
What a Cutoff-Based Result Can and Cannot Tell You
Laid out plainly, a positive result at or above the regulatory cutoff can reliably indicate:
- That THC-COOH is present in the specimen at or above the threshold concentration set in regulation
- That the body has, at some point, metabolized THC
- That the result met the standard for a confirmed positive under a two-tier testing protocol
It cannot reliably indicate:
- Exactly when the substance was used
- How much was used
- Whether the person is currently impaired
- Whether use was a single, isolated event or part of a chronic pattern
This is why the Mandatory Guidelines and DOT's Part 40 framework build in additional layers — a confirmatory test using a more specific analytical method and its own regulatory cutoff, and a review process before any result becomes final — rather than treating the first screening number as the end of the story.
The Review Step Most People Never Hear About
Under DOT-regulated testing, a laboratory result that is positive does not go straight to an employer. It first goes to a Medical Review Officer (MRO) — a licensed physician trained in the interpretation of drug test results — who reviews the result, may contact the individual to ask about legitimate medical explanations, and only then verifies the result as positive, negative, or otherwise, per the process outlined in Part 40. That review step exists precisely because a laboratory number by itself is not a complete picture; it is one input into a verification process designed by regulators who understood its limits.
If you are navigating a testing situation yourself — as an employee, an employer setting policy, or someone trying to understand a result in front of you — the most useful step is usually the same one regulators built into the system: find out whether an MRO review applies, ask what specific test and cutoff was used, and get the confirmatory (not just initial screening) result before drawing conclusions. For anything involving your own employment consequences, a specific legal situation, or a medical explanation for a result, that conversation belongs with your employer's designated MRO, your HR department, or a licensed attorney — not a general explainer article.
Related Reading on HealthDataConsortium.org
For readers researching adjacent questions, HealthDataConsortium.org has published separate, more specific explainers:
- THC Drug-Test Results: What Dilute, Invalid, and Substituted Can Mean — for readers whose lab report used one of those specific terms rather than a straightforward positive or negative
- Will Delta-8 THC Show on a Drug Test? What Labs Actually Detect — for readers asking specifically about delta-8 products rather than standard cannabis use
This article does not cover detection-window estimates or detox methods; those are handled separately because they involve different evidence and different questions than the measurement question addressed here.
Sources
- U.S. Department of Transportation, Office of Drug and Alcohol Policy and Compliance — 49 CFR § 40.85, Cutoff Concentrations for Urine Drug Tests
- American College of Medical Toxicology — Interpretation of Urine for Tetrahydrocannabinol Metabolites (Position Statement)
- Mayo Clinic Laboratories — Marijuana Drug-Class Testing
