Short answer

A urine cannabis test reports nanograms of THC-COOH per milliliter (ng/mL). THC-COOH is the main metabolite the body makes from THC. The screen cutoff is 50 ng/mL. A sample below 50 ng/mL is reported negative, and no second test runs. A sample at or above 50 ng/mL goes to a confirmation test, gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS), with a cutoff of 15 ng/mL. The result is positive only if the confirmation test reads at or above 15 ng/mL. HHS proposed lowering the confirmation cutoff to 10 ng/mL. Confirm the current number with the lab or the program before you rely on it.

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Cutoffs by sample type

  • Urine: 50 ng/mL screen, 15 ng/mL confirmation, measured as THC-COOH.
  • Oral fluid: 4 ng/mL screen, 2 ng/mL confirmation, measured as THC. Federal guidelines set these figures.
  • Hair: picogram-per-milligram cutoffs. Labs set their own limits, and 0.1 ng/mg and 1 pg/mg are common thresholds for different analytes.
  • Blood: labs often report THC down to 1 to 2 ng/mL in whole blood. There is no single national cutoff for private blood tests.

Employers who are not covered by federal rules set their own cutoffs. Some use 20 ng/mL or 15 ng/mL for the urine screen. A lower screen cutoff means more samples move to confirmation.

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Detection windows

Urine detection depends on dose, potency, and how often a person uses. A single use is detectable for about 1 to 3 days. Daily use is detectable for 5 to 10 days. Chronic heavy use is detectable for up to 30 days in published cases, and longer in a small number of reports. THC is stored in fat tissue and released as fat breaks down, so the window does not track how recently someone used.

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Blood detects THC for hours, not days. Oral fluid detects THC for about 24 to 48 hours after use. Hair grows about 1 cm per month, and a 3.9 cm sample covers roughly 90 days.

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What moves the number

  • Dose and product potency.
  • Frequency of use and years of use.
  • Body fat percentage.
  • Metabolism and liver enzyme activity.
  • Water intake before the sample.
  • Exercise and diet, which release stored THC.

None of these factors change the cutoff. They change the concentration in the sample.

Dilute samples and validity tests

Drinking large amounts of water lowers THC-COOH per milliliter and can push a sample under 50 ng/mL. Labs test for this. A urine creatinine below 20 mg/dL is flagged as dilute under federal rules. Creatinine below 5 mg/dL, or specific gravity outside set limits, marks the specimen as invalid. Labs also check pH and look for oxidants and other adulterants. A dilute or invalid specimen is not a negative result. It is a separate finding, and many programs treat it as a refusal or require a new sample.

Cross-reactivity and false positives

Immunoassay screens can bind to compounds other than THC-COOH. Reported cases include ibuprofen, naproxen, pantoprazole, efavirenz, and some antihistamines. The confirmation test resolves these. GC-MS and LC-MS/MS identify the molecule, so a cross-reactive drug does not produce a confirmed positive.

CBD products can contain trace THC. High-dose oral CBD has produced positive screens in a small number of case reports. Secondhand smoke in a ventilated room stays below 50 ng/mL in most studies. Enclosed spaces with heavy smoke, such as a small car, raise exposure and raise the chance of a positive at lower cutoffs.

Law and testing at work

Federal DOT testing covers truck drivers, pilots, rail workers, and others in safety-sensitive roles. Cannabis stays on the DOT panel, and a confirmed positive is a violation. Several states limit pre-employment cannabis testing for off-duty use. California AB 2188 took effect January 1, 2024. Nevada, New York, New Jersey, and Washington passed similar measures. Exemptions cover federal contractors, DOT positions, construction, and jobs that require testing under federal or state law. Rules change, so check the current statute for your state.

What a cutoff does not measure

A cutoff is a detection limit, not a measure of impairment. Urine THC-COOH can stay above 15 ng/mL for weeks after effects end. A person with a confirmed positive result may have used days or weeks earlier. Blood THC reflects recent use, but impairment research does not support a fixed blood number that applies to all users.