Blood THC After Requested Abstinence: Can One Result Date Cannabis Use?
A single positive blood THC result cannot reliably date a cannabis-use episode. In a study of 190 regular users asked to abstain for at least 48 hours, substantial numbers still had measurable THC before the experimental dosing session.
What “abstinence” meant in this study
Fitzgerald and colleagues analyzed baseline whole-blood concentrations in participants who had received an abstinence instruction. Their self-reported interval since last use had a median of three days. This was not continuously observed inpatient abstinence, so the instruction and report should not be described as proof of each person’s exact last-use time.
The distinction preserves the study’s value without overstating it: it documents what investigators encountered in a defined sample under a specified preparation protocol.
The baseline numbers challenge simple timing claims
At baseline, 43% had THC concentrations of at least 0.5 ng/mL, 24% had at least 2 ng/mL, and 5.3% had at least 5 ng/mL. The maximum baseline value was 16.2 ng/mL.
These are whole-blood findings. The values should not be directly transferred to plasma or serum, and the cutpoints used in the analysis should not be presented as a statement of the applicable law in any particular jurisdiction.
Baseline simulator performance did not differ significantly when participants were divided using the examined concentration cutpoints. That null comparison does not establish that every participant was unimpaired, that THC never affects driving, or that all higher concentrations have the same meaning.
What an expert can reasonably conclude
The study makes a categorical claim such as “this concentration proves use immediately before driving” difficult to justify from concentration alone. It does not provide a formula that can reconstruct the last-use time for a new individual.
Relevant additional evidence includes the actual matrix, cannabis-use history, collection interval, analytical quality and independently documented behavior. Those questions belong alongside the distinction between exposure, functional impairment and crash causation. The research supplies evidence about the limitations of an inference; it does not decide the facts of a particular case.
Primary research
Per Se Driving Under the Influence of Cannabis Statutes and Blood Delta-9-Tetrahydrocannabinol Concentrations following Short-Term Cannabis Abstinence. DOI: 10.1093/clinchem/hvaf121.
