Laboratory Studies
Urine immunoassays are used to detect a conjugated delta-9-tetrahydrocannanbinol (THC) metabolite, 11-nor-delta-9-THC carboxylic acid. Features of the test results are as follows [23] :
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After a single, acute use, THC is detectable in the urine for 7 days
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With chronic use, THC is detectable in the urine for 10-30 days
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Enzyme-multiplied immunoassay technique (EMIT) can detect urine levels as low as 20-100 ng/mL.
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These screening tests correlate with prior exposure, not with the amount used or degree of clinical effect.
False-positive results can occur with ibuprofen, naproxen, dronabinol, efavirenz, and hemp seed oil. False-positive results are unlikely to result from use of health food store hemp products or from second-hand smoke inhalation, unless this exposure occurs in an unventilated space.
Quantitative assays for 11-nor-delta-9-THC carboxylic acid are available in most laboratories, but the assay must be specifically requested. Blood tests can also be used to detect THC; however, the levels correlate poorly with the clinical effects.
Hair sampling tests have become available using gas chromatography and mass spectrometry assays and can test for multiple cannabinoids, including THC, THC-OH, THC-COOH, cannabinol and CBD. Cannabinoids enter the hair through capillaries and sweat and can be detected up to 3 months after exposure. However, detection depends on heaviness of use and potency of marijuana consumed. For heavy, near daily smokers, THC was detected in hair with a sensitivity of 0.77 but this number falls to 0.55 when looking at all cannabis users. [24]
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Flowering top of cannabis plant.
