Quick answer
A cannabis drug test does not look for THC itself. It looks for THC-COOH, the metabolite your liver creates after you consume cannabis. Results are reported in nanograms per milliliter (ng/mL), and every test has a cutoff: the concentration at or above which a sample is called positive. Under federal workplace guidelines, the urine screening cutoff is 50 ng/mL and the confirmatory cutoff is 15 ng/mL.
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What a cutoff level actually is
A cutoff is a detection threshold, not a measure of impairment. A sample that reads 48 ng/mL on the screen is reported negative even though THC metabolites are present. A sample that reads 60 ng/mL is not automatically positive either. It moves to a second, more precise test, and only that confirmatory result decides the outcome.
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Cutoffs exist because immunoassay screens can react to other compounds in urine. Setting the screen at 50 ng/mL keeps false positives low while still catching regular cannabis use.
Standard cutoff levels by test stage
- Initial immunoassay screen: 50 ng/mL for THC-COOH.
- Confirmatory GC-MS or LC-MS/MS: 15 ng/mL for THC-COOH.
- Many private employers, labs, and at-home kits use lower screening cutoffs of 20 ng/mL or 15 ng/mL.
- Hair, saliva, and blood panels set their own cutoffs, so a number from one test type does not transfer to another.
How a urine cannabis test is processed
- The collector seals the sample and splits it into an A bottle and a B bottle.
- The lab runs an immunoassay screen against the 50 ng/mL cutoff.
- Any sample at or above the screen cutoff goes to confirmatory testing by gas chromatography mass spectrometry or liquid chromatography tandem mass spectrometry at 15 ng/mL.
- A medical review officer checks the confirmed result against prescription records and interviews the donor before releasing a final report.
- The B bottle is stored so the donor can request a retest of the same specimen.
Detection windows by sample type
- Urine: about 1 to 3 days after one-time use, and up to 30 days or longer with heavy daily use.
- Blood: roughly 1 to 2 days, since THC clears the bloodstream quickly.
- Saliva: about 24 to 48 hours after the last use.
- Hair: up to 90 days, because metabolites become trapped in the hair shaft as it grows.
What changes how long THC stays detectable
- Frequency of use. Chronic daily users store more metabolite in fatty tissue and release it slowly.
- Potency and dose. High-THC concentrates deliver a larger metabolite load than flower.
- Body fat percentage. THC-COOH is fat soluble and lingers longer in people with more adipose tissue.
- Metabolism and age. Faster metabolizers clear the metabolite sooner.
- Hydration and exercise. These shift water balance and can move stored metabolite back into circulation.
Dilution and validity checks
Labs test every urine sample for creatinine, specific gravity, and pH. A creatinine reading below 20 mg/dL, along with low specific gravity, flags the specimen as dilute. A dilute sample can be rejected as invalid rather than reported as negative, and the donor may be asked to submit a new sample under observed collection.
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State legality and testing
State-legal cannabis use does not change federal testing rules. Employers in legal states can still screen for THC, and a positive result at or above the cutoff is a positive result regardless of where the cannabis was purchased. A few states limit how employers may use pre-employment marijuana results, so check your own state law and your employer policy.