Researchers from the University of British Columbia have found that tetrahydrocannabinol (THC), the psychoactive compound in cannabis, can be detected in the body for up to a month after consumption. As such, they say that cannabis blood tests are not reliable in saying whether someone has just used the substance or is under its influence.
For the study, the researchers conducted a systematic review of existing literature on residual blood THC concentrations in frequent cannabis users after a period of abstinence. Of 1612 articles initially identified for review, they ended up including six studies in their analysis- the only studies that met their inclusion criteria.
All in all, the researchers found that THC concentrations of 2ng/ml and above could linger for extended periods of time in the bloodstream, and far beyond time periods in which a person could be under its influence.
In particular, the researchers noted that THC levels among some participants remained above 2ng/ml several days after abstinence- with some testing positive for up to 30 days. Some participants even displayed a ‘double bump’ pattern in which their THC levels rose towards the end of the week after declining. Others maintained THC levels above 5ng/ml a day after abstinence.
As such, the researchers say that measuring THC levels in the blood is not a fair assessment of recent exposure to cannabis and can not reliably indicate whether someone has recently been using the substance and is still feeling its effects.
The researchers thus say that law enforcers should refrain from using THC blood to uphold road safety laws, which often have a zero-tolerance to cannabis use. To properly uphold these laws without incriminating innocent individuals, they recommend more research to be conducted into more reliable ways to assess whether people are under the influence of cannabis at a given moment.