Still Testing Positive: Implications of Persistent Blood THC Concentrations in Regular Cannabis Users
April 24, 2025
At the April CMCR Investigators’ Meeting, Ray Suhandynata, Ph.D., DABCC, Assistant Professor at UC San Diego in the Department of Pathology and Skaggs School of Pharmacy and Pharmaceutical Sciences, presented his ongoing research investigating the implications of THC blood levels on driving impairment among regular cannabis users. A total of 191 users were randomized into three groups—0% THC (placebo), 5.9% THC, and 13.4% THC—and assessed through driving simulations, blood/saliva analyses, and standardized field sobriety tests. Baseline THC concentrations varied, with frequent users showing higher residual THC even after 48-hour abstention. Results indicated no relationship between immediate post-smoking blood THC levels and driving performance. Importantly, many regular users exceeded “per se” DUI cannabis (DUIC) thresholds, including 40% surpassing zero-tolerance limits and nearly 25% exceeding the 2 ng/mL cutoff days after last use. Despite these elevated baseline THC levels, driving simulation scores were unaffected. The data challenge the validity of current DUIC “per se” statutes, suggesting they may lead to unwarranted impairment charges. Incorporating additional toxicological measures, such as oral fluid THC assays, significantly enhanced classification accuracy, reducing false positives. Future research will explore impairment effects using contemporary cannabis products (e.g., concentrates).
Ray Suhandynata, Ph.D., DABCC
Dr. Suhandynata is an Assistant Professor at UC San Diego in the Department of Pathology and Skaggs School of Pharmacy and Pharmaceutical Sciences. He serves as the Associate Director of the CMCR Reference Laboratory and provides clinical oversight for Clinical Chemistry and Clinical Toxicology laboratories at UC San Diego Health. His research includes developing mass spectrometry applications in forensic, pre-clinical, and clinical laboratories, focusing particularly on enhancing the predictive value of forensic toxicology in officer-performed field sobriety testing.