Vancouver, Canada: The detection of THC in blood at levels greater than 2ng/ml may persist for extended periods of time and therefore it is not necessarily indicative of recent cannabis exposure, according to data published in the journal Drug and Alcohol Dependence.
Investigators affiliated with the University of British Columbia performed a systematic review of the relevant literature assessing residual THC plasma levels in frequent cannabis consumers.
Authors reported: “[I]n all studies where participants were observed for over a day, blood THC [levels] in some participants remained detectable during several days of abstinence,” with some subjects continuing to test positive for up to 30 days. Some subjects also demonstrated a so-called “double hump” pattern “where their THC levels rose toward the end of the week after an initial decline.”
Researchers concluded, “The studies in our review consistently demonstrate that positive blood THC levels, even levels over 2ng/ml, do not necessarily indicate recent cannabis use in frequent cannabis users.”
The study’s findings have implications for traffic safety laws, as several US states impose either per se or zero-tolerant per se laws that criminalize the operation of a vehicle by a driver solely based upon the detection of trace levels of THC in one’s blood. NORML opposes the imposition of per se limits for cannabis because the presence of THC in blood, especially at low levels, is not a consistent predictor of either recent use or impairment of performance.
Full text of the study, “Residual blood THC levels in frequent cannabis users after over four hours of abstinence,” appears in Drug and Alcohol Dependence. Additional information is available from the NORML fact sheet, “Marijuana and Psychomotor Performance.”
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