Oral fluid THC was the best biomarker for detecting cannabis use within 3 hours

Oral fluid THC at 10 ng/mL cutoff showed 99.7% specificity, 82.4% sensitivity, 92.5% positive predictive value, and 99.2% negative predictive value for detecting cannabis use within 3 hours. Frequent users had residual THC in blood but not oral fluid before smoking, making oral fluid less prone to false positives from prior-day use. Blood CBN at 1 ng/mL had 100% positive predictive value but only 31.4% sensitivity.

Randomized trial with 191 frequent and occasional cannabis users who smoked placebo, 5.9%, or 13.4% THC cannabis. Blood, oral fluid, and breath collected before and up to 6 hours after smoking. Receiver operating characteristic analysis identified optimal biomarker cutoffs for recent use detection.

Proving cannabis-impaired driving requires a biomarker that can reliably distinguish recent use from residual detection. This study provides the most comprehensive comparison of blood, oral fluid, and breath for this purpose.

No biomarker equates to impairment, but oral fluid THC comes closest to identifying recent intake. The lack of residual oral fluid THC in frequent users (unlike blood) makes it a fairer testing medium that does not penalize chronic users for prior-day use.

Only smoked cannabis tested. Oral fluid THC at 10 ng/mL still detectable in 23% at 4.4 hours, limiting late specificity. Cannabis impairment does not correlate linearly with THC concentrations. Lab setting may not reflect real-world conditions.