Adding the Finger-to-Nose test and head movement observations to the standard field sobriety test increased detection of cannabis-impaired participants from 67% to 88% at a mean THC of 6.34 ng/mL.
Law enforcement, traffic safety researchers, DRE-certified officers, driving policy makers
Sensitivity improved from 0.67 to 0.88
What the researchers found
Twenty minutes after vaping cannabis (mean THC 6.34 ng/mL), 67% met SFST criteria for suspected impairment. Adding the Finger-to-Nose test plus head movement/jerk observations increased detection by 33%, improving sensitivity from 0.67 to 0.88.
Why it matters
The standard field sobriety test was designed for alcohol and misses a third of cannabis-impaired individuals. Enhancing the test could significantly improve road safety without requiring blood tests.
The numbers in context
Mean THC: 6.34 ng/mL at 20 minutes post-vaping. SFST alone: 67% detected. SFST + FTN + HMJ: 88% detected. Sensitivity improved from 0.67 to 0.88 (33% increase).
How the study worked
Observational study where participants used their own cannabis at a research facility. SFST plus supplementary tests administered by certified Drug Recognition Experts at baseline and four times during 150 minutes post-use. Physiological indicators, vital signs, and digit-symbol substitution also assessed.
What this study cannot tell us
Participants knew they were being tested (no blinding). Assessors knew participants had used cannabis. Observational design in a controlled setting, not actual roadside conditions. Small sample. Voluntary cannabis use, not dose-controlled.
How to read the evidence
Observational study with non-blinded assessors provides preliminary evidence but needs validation in randomized and field settings.
When this study was published
2024 observational study of enhanced field sobriety testing for cannabis
The bigger picture
As cannabis legalization spreads, law enforcement needs practical, roadside tools to detect cannabis impairment. Unlike alcohol breathalyzers, there is no quick cannabis impairment test. Enhancing existing field sobriety assessments is a pragmatic approach while technology develops.
Questions still open
- Would these results replicate in blinded, randomized trials? How does the enhanced SFST perform at lower THC levels or with edibles? What is the false-positive rate in sober individuals?
Common questions
Can police detect cannabis impairment without a blood test?
Why is detecting cannabis impairment harder than alcohol?
Read the original research
Enhancing the Standardized Field Sobriety Test to detect cannabis impairment: An observational study.
Traffic injury prevention, 25(1), 1-7
Citation
Beirness, Douglas J; Smith, D'Arcy; Brubacher, Jeff R. (2024). Enhancing the Standardized Field Sobriety Test to detect cannabis impairment: An observational study.. Traffic injury prevention, 25(1), 1-7. https://doi.org/10.1080/15389588.2023.2262658