Before experimental dosing, participants supplied urine until the immunoassay result was below 20 ng/mL. That urinary criterion did not establish that their existing hair contained no cannabinoids from earlier use.
The study used a matrix-specific entry sequence
Participants initially qualified with a positive urine cannabinoid screen and a documented cannabis-use history. After admission to a secure unit, daily urine testing continued until the specified lower screen threshold was reached, taking nine to twenty-four days. Hair was collected before and after the research administrations.
The urinary measurement and the hair measurement sampled different biological materials with different histories and reporting criteria.
A threshold is not a universal clearance marker
Falling below one urine immunoassay threshold does not certify chemical absence in all tissues or eliminate a pre-existing hair record. It also does not precisely date the last cannabis use. The threshold was a protocol condition preceding dosing, not a validation of drug-free hair.
Why baseline history remains relevant
The study’s post-dose hair findings need to be read alongside prior use and the original hair collection. They cannot be attributed solely to the most recent controlled administration by assuming the intervening urinary decline reset the hair. The same caution applies when a later negative urine screen is used to make claims about an earlier period represented by hair.
Related reading: interpreting cannabinoid assay validation.
Research cited
Huestis and colleagues (2006). Cannabinoid Concentrations in Hair from Documented Cannabis Users. DOI: 10.1016/j.forsciint.2006.08.005.
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