The National Highway Traffic Safety Administration issued a report to Congress in July 2017 about marijuana impaired driving. The report noted that studies have consistently found that the level of THC in the blood and the degree of impairment do not appear to be closely related. “While high levels of THC are detected in the blood (and oral fluid) during and right after smoking, they are not typically observed an hour or two later. In cases of traffic crashes or arrests for impaired driving, it is most likely that only relatively low levels of THC will be found by the time an oral fluid or blood sample is obtained. Low THC levels of a few nanograms per milliliter (ng/ml) in blood can result from relatively recent use (e.g., smoking within 1 – 3 hours) when some slight or even moderate impairment is likely to be present, or it can result from chronic use where no recent ingestion has occurred and no impairment is present.” Thus, there is a lack of clear correspondence between THC level in plasma and impairment. The peak THC level is reached soon after smoking ends, but peak performance deficits are observed long after the peak THC level occurs. The subjectively reported “high” also does not correspond well with blood plasma THC concentration. THC level in blood (or oral fluid) does not appear to be an accurate and reliable predictor of impairment from THC. Also, when low levels of THC are found in the blood, the presence of THC is not a reliable indicator of recent marijuana use.
The full report is available here.