A urine immunoassay for cannabinoids returns positive in a man arrested for driving under influence. He admits smoking cannabis once at a party three weeks ago. Which statement best explains this result?

• A Delta-9-tetrahydrocannabinol itself remains detectable in urine for weeks because it is water soluble
• B A single exposure cannot produce a positive urine test beyond 48 hours, so the assay must be a false positive
• C Cannabinol is stored in bone marrow and released over months, prolonging urinary positivity
• D The lipophilic metabolite 11-nor-9-carboxy-delta-9-tetrahydrocannabinol accumulates in adipose tissue and is released slowly, remaining detectable in urine for weeks in chronic users ✓

Explanation

THC is highly lipophilic and its inactive carboxy metabolite, 11-nor-9-carboxy-THC, deposits in adipose tissue from where it is slowly released, so urine remains positive for several days in occasional users and up to four to six weeks or longer in chronic users. Parent THC is not what is measured, killing option A, and even a single heavy exposure can exceed 48 hours of positivity, killing option B.

Reference: Casarett and Doull's Toxicology: The Basic Science of Poisons, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.