Neither the detection of THC nor its metabolites in blood or saliva is predictive of impaired performance, according to a meta-analysis of the relevant literature published in the journal Neuroscience and Behavioral Reviews.
Australian researchers reviewed the results from 28 eligible studies (involving 824 participants) that assessed the relationship between THC-related biomarkers and individuals’ performance.
They reported that no consistent relationship exists between THC or THC-metabolite concentrations and impaired performance, and that such relationships are nearly impossible to infer in more habitual cannabis consumers. Further, they identified blood THC concentrations as “the poorest correlates of impairment, demonstrating a ‘very weak’ relationship after both ingestion and inhalation of THC.”
Authors also acknowledged that the imposition of so-called per se limits for the presence of THC in blood is ill advised because the laws are “unlikely to be effective in distinguishing between impaired and unimpaired (or not meaningfully-impaired) regular cannabis users” who may have residual levels of THC or its related metabolites for extended periods of time absent any actual impairment.
Per se traffic laws make it a crime for a driver to operate a motor vehicle with trace levels of either THC or its metabolites in their blood or urine, regardless of whether or not there exists any demonstrable evidence that the driver is under the influence.
Five states – Illinois, Montana, Ohio, Pennsylvania, and Washington – impose per se limits for the presence of specific amounts of THC in blood while 11 states (Arizona, Delaware, Georgia, Illinois, Iowa, Michigan, Oklahoma, Rhode Island, South Dakota Utah, and Wisconsin) impose zero tolerant per se standards. In those states, it is a criminal violation of the traffic safety laws to operate a motor vehicle with any detectable levels of THC in blood. (Some of these states make exceptions for medical marijuana registrants.) Colorado law infers driver impairment in instances where THC is detected in blood at levels of 5ng/ml or higher.
Last year, lawmakers in two states – Indiana and Nevada – enacted legislation repealing elements of their per se traffic safety laws. In Arizona, the state’s Supreme Court struck down a traffic safety law that made it a per se violation to operate a vehicle with trace concentrations of inert THC metabolites, but they left in place similar restrictions for the presence of THC.
NORML has long opposed the imposition of THC per se thresholds for cannabinoids in traffic safety legislation, opining: “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.”
The Australian scientists issued a similar conclusion, finding: “Results indicate that blood THC, 11−OH-THC and 11−COOH-THC concentrations, oral fluid THC concentrations, and subjective ratings of intoxication are relatively poor indicators of cannabis-induced impairment. The use of per se limits as a means of identifying cannabis-impaired drivers should therefore be re-considered.”
Several traffic safety agencies, including the US National Highway Traffic Safety Administration (NHTSA) and AAA, have similarly recommended against the use of per se limits for THC and its metabolites.
Full text of the study, “Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis,” appears in Neuroscience and Behavioral Reviews. Additional information is available from the NORML fact sheet, “Marijuana and Psychomotor Performance.” NORML’s white paper addressing per se limits is available online.