Combining THC Testing With Field Sobriety Changes the Classification Question
Requiring both a positive THC test and field-sobriety findings can reduce the number of placebo participants classified as impaired. That does not establish that the combined rule accurately measures every individual’s driving ability.
A controlled study with several distinct references
The research program randomized 191 participants to smoke placebo or cannabis containing 5.9% or 13.4% THC. Investigators collected blood, oral fluid and breath, measured simulator outcomes and obtained field-sobriety assessments from officers.
Assignment to active cannabis, detectable THC and reduced simulator performance are different reference conditions. A rule that better distinguishes the assigned treatment groups is not automatically a validated test of impairment.
Adding a cutoff changes who qualifies
The paper examined combining field-sobriety findings with blood or oral-fluid THC requirements. Adding a chemical requirement excluded some placebo participants who otherwise met the field-sobriety classification, but the choice of cutoff and sampling time affected the result.
Separately, THC concentrations in the measured biofluids did not significantly correlate with lane-position variability or car-following coherence at the studied time points. This null relationship limits claims that a concentration alone quantifies the functional outcome.
Use the corrected evidence and the appropriate inference
A published correction amended several field-sobriety-only entries in Supplemental Table 3. Numerical applications of that table must use the corrected file. The correction is available at DOI 10.1093/clinchem/hvad218.
The study used a simulator and selected cannabis-experienced participants. Related publications from this trial are additional analyses rather than independent replications. For a case-specific opinion, identify the classification target, elapsed time and actual functional evidence before treating a combined rule as proof of an individual’s impairment.
Related: THC concentration and impairment.
Primary research
Driving Under the Influence of Cannabis: Impact of Combining Toxicology Testing with Field Sobriety Tests. Clinical Chemistry (2023).
