High Serum THC in a Tourette Case: Why One Report Cannot Set a Driving Rule
A case report describing satisfactory psychometric performance despite high serum THC challenges a simple concentration-only explanation, but it does not establish a safe THC concentration. Its value depends on preserving the patient’s unusual clinical circumstances and the limits of the testing.
What the report described
Streetz and colleagues assessed a 28-year-old man with Tourette syndrome and ADHD who had used substantial amounts of medicinal cannabis for years. Testing occurred on a day with cannabis use and on the next day without additional use before testing. The investigators used the Vienna Test System and clinical assessments, with serum cannabinoid measurements.
The reported serum THC reached 364 ng/mL on the use day. The patient met the paper’s psychometric criterion for fitness to drive at the assessed times. This is a human case report involving one selected patient, not a randomized trial or a representative sample of drivers.
Serum is not whole blood
The reported concentration belongs to serum. It should not be relabeled as a whole-blood value, nor converted using an assumed fixed ratio. The study does not calibrate any concentration as a universal marker of safety or impairment.
The untreated comparison was missing
The patient was not assessed after a sufficiently established drug-free period. Residual THC remained measurable on the second day. Consequently, the report cannot determine his driving-related function without treatment, prove that cannabis improved it, or isolate tolerance from symptom control and other explanations.
Repeated testing also raises practice effects. The authors acknowledge that on-road assessment is superior to the psychometric battery for predicting actual driving ability. Meeting a study’s testing criterion is not equivalent to demonstrating safe performance in every traffic situation.
What an expert can reasonably take from it
The case illustrates the danger of treating one serum concentration as a complete description of a person’s function. It does not justify recommending the patient’s dose to others, applying foreign legal commentary to a California matter, or treating medicinal status as automatic driving clearance.
A useful opinion must connect the individual’s condition, treatment history, timing, matrix, observed performance, and alternative explanations. The report supports individualized scientific analysis; it cannot replace that analysis.
Related evidence
See our related analysis of toxicology evidence and its limits.
Primary research
Case Report: Effect of medicinal cannabis on fitness to drive in a patient with Tourette Syndrome and ADHD. Frontiers in Psychiatry, 2025. DOI: 10.3389/fpsyt.2025.1595649.
