Oral Fluid Screening and Blood Tests for THC in Suspected Cannabis-Impaired Drivers

The research, published in the Journal of Analytical Toxicology, directly evaluated the field performance of Canada’s Approved Drug Screening Equipment (ADSE). Following legalization, law enforcement required a scientifically valid tool to identify drivers likely exceeding the strict per-se blood THC limits of 2 ng/mL and 5 ng/mL. The approved Drager DrugTest 5000 and Abbott SoToxa devices use an oral fluid drug test with a standardized cutoff of 25 ng/mL THC to provide the necessary grounds for further action.

The study’s strength lies in its analysis of real-world data from 230 drivers who screened positive at the roadside. Key procedural and demographic details include:

• A confirmatory blood sample was collected an average of 1.4 hours later for precise laboratory analysis using UPLC-MS-MS, the forensic gold standard.
• Most stops were for observable traffic infractions like speeding (43.8%) or routine spot-checks (21.7%), demonstrating that cannabis-impaired driving is not always visibly obvious.

A significant majority (81%) of drivers admitted to recent cannabis use when confronted with a positive screen, adding behavioural context to the chemical evidence.

The findings offer powerful, quantitative support for the oral fluid drug test as a reliable indicator. Critically, this high correlation held despite the inherent delay between the roadside screen and the blood draw:

• 90% Correlation: Nine out of ten drivers who failed the oral screen had a subsequent blood THC level at or above the 2 ng/mL legal limit.
• 70% Correlation: Seven out of ten tested at or above the stricter 5 ng/mL limit, which carries more severe penalties.

These results decisively validate the oral fluid drug test as a highly effective real-time filter. The data strongly indicates that a positive roadside result is a probable indicator of both recent use and a blood concentration likely to be illegal, not a mere trace detection.

The study’s conclusions provide clear, actionable guidance for 2024 enforcement strategies. A positive oral fluid drug test provides a scientifically backed, practical basis for forming the “reasonable grounds” required under Canadian law to demand an evidentiary blood test. This procedural step is crucial because the blood test result—not the oral fluid screen—serves as the primary evidence for a per-se criminal offence.

For officers in the field, this validation translates to:

• Operational Efficiency: The screen allows for quicker, more confident decision-making at the roadside.
• Enhanced Objectivity: It provides an empirical, numerical result that supplements traditional physical sobriety observations.

Legal Defensibility: The documented 90% correlation rate supports the test’s use in court to justify the subsequent blood draw, strengthening the chain of evidence.

The proven reliability of the oral fluid drug test has significant implications beyond roadside traffic enforcement. Its non-invasive nature and validated accuracy make it a compelling tool for other safety-sensitive scenarios:

• Post-Incident Workplace Testing: For investigations in industries like transportation, construction, or manufacturing following an accident or near-miss.
• Reasonable-Cause Screening: Enabling employers in jurisdictions where permitted to test based on specific, articulable observations of impairment.
• Deterrence and Safety Culture: Providing a visible, evidence-based tool that reinforces a company’s commitment to a drug-free workplace, aligning field screening with forensic confirmation.

The 2024 study solidifies the role of the oral fluid drug test as a key component in a modern, science-based framework for deterring and detecting cannabis impairment across multiple domains of public and occupational safety.

Reference

Copley, H., Filbert, A., and Williams, K., “Evaluation of the Canadian Approved Drug Screening Equipment Cut-Off Levels for Tetrahydrocannabinol (THC)”, Journal of Analytical Toxicology, 48: 165-170, 2024