Blood samples may be used to measure quantitative levels of THC. Blood analysis is the preferred method of detection for interpretation of acute effects.
Cannabinoids can be detected in the urine for as many as 21 days after use in persons chronically using marijuana because these lipid-soluble metabolites are slowly released from fat cells into the blood; however, 1-5 days is the normal urine-positive period. Urine immunoassays correlate with prior exposure, not with the amount used or degree of clinical effect.
With laboratory tests for cannabis use, false-negative results tend to be more common than false-positive results.
For more on laboratory findings associated with marijuana use, read here.
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Cite this: Sage W Wiener, Michael E Schatman. Fast Five Quiz: Marijuana--Medical Uses and Personal Abuses - Medscape - May 24, 2017.