Sweat-Patch THC Could Remain Positive During Monitored Abstinence
A positive THC sweat patch did not invariably indicate new cannabis use during the week it was worn. A controlled abstinence study documented continued positivity after cessation, while a separate oral-dosing experiment produced negative patches despite known THC administration.
Weekly patches measured accumulated material
Investigators followed 11 daily cannabis users during continuously monitored abstinence. Seven-day patches were analyzed for parent THC, with a quantification limit of 0.4 ng per patch. Results were also classified using a 1 ng-per-patch decision cutoff examined in the paper.
Ten of 11 first-week patches exceeded that decision cutoff. Positivity declined in later weeks, but one of the five participants remaining at week four still exceeded it. The changing denominator matters: this was not evidence that every participant was followed for four weeks.
Known oral dosing did not guarantee a positive patch
A separate group of seven received oral THC regimens of up to 14.8 mg per day for five consecutive days. No daily or weekly patch contained quantifiable THC. Concurrent plasma concentrations were relatively low under this protocol.
The contrast demonstrates route- and protocol-dependent detection. It does not prove that every oral exposure will be missed, or that a patch is incapable of detecting THC under different conditions.
Interpret the wear interval and cutoff together
A patch summarizes deposition across its wear period rather than recording an instantaneous concentration. Continued excretion from earlier use can therefore complicate attribution of a positive patch to a new event.
The paper’s historical proposed workplace cutoff should not be presented as a current legal requirement. For a case-specific assessment, identify the actual laboratory cutoff, wear dates, previous use and route. Neither a positive patch nor a negative patch establishes a person’s functional capacity at a particular time.
Related: Why cannabinoid concentration and impairment require separate interpretation.
Primary research
Excretion of Δ9-Tetrahydrocannabinol in Sweat. Forensic science international (2007).
