Providence, RI: The detection of THC in biological fluids is not predictive of psychomotor impairment, according to a literature review published in the journal Current Addiction Reports.
Researchers at Brown University affirmed: “There are no reliable or practical biochemical or behavioral methods used in real-time with drivers on the road to determine cannabis-induced impairment. … Many studies have found weak or non-existent correlations between THC concentrations in blood, oral fluid, or breath and actual driving performance or impairment.”
That finding is consistent with the opinions of numerous scientists and traffic safety groups, including the National Highway Traffic Safety Administration and the American Automobile Association.
The study’s authors concluded: “These findings collectively underscore that THC concentrations in common biofluids (e.g., blood and saliva) and exhaled breath are unreliable as sole indicators of current driving impairment. … There are no empirically supported thresholds for blood or oral fluids that reliably indicate cannabis impairment.”
Nonetheless, several states impose per se limits for motorists who are determined to have trace levels of THC in their blood or other bodily fluids. (These laws criminalize operating a motor vehicle with detectable quantities of THC or its metabolites, even absent evidence of driving impairment.) Several studies have determined that subjects may continue to test positive for traces of THC in their blood and oral fluids for days post-abstinence.
NORML has long opposed the imposition of per se THC limits for motorists and has alternatively called for the expanded use of mobile performance technology like DRUID. In a peer-reviewed paper published by the Humboldt Journal of Social Relations, NORML’s Deputy Director Paul Armentano wrote: “The sole presence of THC and/or its metabolites in blood, particularly at low levels, is an inconsistent and largely inappropriate indicator of psychomotor impairment in cannabis consuming subjects. … Lawmakers would be advised to consider alternative legislative approaches to address concerns over DUI cannabis behavior that do not rely solely on the presence of THC or its metabolites in blood or urine as determinants of guilt in a court of law. Otherwise, the imposition of traffic safety laws may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish those who have engaged in legally protected behavior and who have not posed any actionable traffic safety threat.”
Full text of the study, “Recent advances in the science of cannabis-impaired driving,” appears in Current Addiction Reports. Additional information is available from the NORML Fact Sheet, ‘Marijuana and Psychomotor Performance.’