Short answer
Urine immunoassay screenings detect carboxy-THC at a 50 ng/mL cutoff. Studies report sensitivity and specificity above 90 percent at that level. Confirmatory tests using GC-MS or LC-MS/MS are close to 100 percent accurate for the same metabolite at a 15 ng/mL cutoff. Accuracy drops with dilute samples, early collection, and cross-reactive medications.
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Screening vs. confirmatory tests
Most workplace tests use a two-step process. Step one is an immunoassay. It binds antibodies to THC metabolites. Step two is gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS). A positive screening result is not final. A lab runs the confirmatory test on the same sample. Only the confirmatory result is reported as positive.
Immunoassays are fast and cheap. They also react to compounds with similar structures. Confirmatory tests identify the exact molecule. They do not cross-react.
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Cutoff levels
The federal workplace drug testing program sets cutoffs for urine. The screening cutoff for THC-COOH is 50 ng/mL. The confirmatory cutoff is 15 ng/mL. A sample below 50 ng/mL is reported as negative, even if THC is present. A sample between 15 and 50 ng/mL is negative on the screening test but would be positive on a GC-MS test.
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Detection windows
Detection time depends on the specimen, dose, potency, frequency of use, body fat, and metabolism.
- Urine: 1 to 3 days after one use. Up to 30 days or more for heavy daily users.
- Blood: 3 to 12 hours for active THC. Up to 1 to 2 days for carboxy-THC.
- Oral fluid: 24 to 48 hours after last use.
- Hair: up to 90 days. Not part of the federal workplace program.
What causes false positives
Some prescription drugs cross-react with urine immunoassays. Examples include efavirenz (an HIV drug), high-dose ibuprofen, naproxen, pantoprazole, and promethazine. Dronabinol (Marinol) is synthetic THC and will test positive. Full-spectrum CBD products can contain enough THC to trigger a positive result in some users. A confirmatory test resolves these cases.
What causes false negatives
Dilute urine is the most common cause. Drinking large amounts of water lowers creatinine and specific gravity. Labs flag samples with creatinine below 20 mg/dL as dilute. A dilute sample may be rejected or reported as negative. Testing too early after use, or after a long gap, also produces false negatives. Some people metabolize THC faster than others.
Secondhand smoke and CBD
Secondhand cannabis smoke can raise metabolite levels. Controlled studies with extreme exposure in small, unventilated rooms produced detectable carboxy-THC. Levels stayed below the 50 ng/mL cutoff in most cases. Occasional exposure in a ventilated space is unlikely to cause a positive result. CBD isolate does not contain THC. CBD full-spectrum oils may contain trace THC. Daily high-dose use can accumulate THC metabolites.
Summary
Urine immunoassays are accurate for detecting heavy or recent cannabis use. They are less reliable near the cutoff and for occasional use. Confirmatory GC-MS or LC-MS/MS tests are near 100 percent accurate. Detection windows run from hours for blood to weeks for chronic urine users. False positives come from cross-reactive drugs and some CBD products. False negatives come from dilute urine and timing.