Oral-Fluid THC Device Errors Depended on the Reference Cutoff

Two oral-fluid THC screening devices produced both false-negative and false-positive classifications when compared with laboratory measurement after controlled vaporization. Those terms describe disagreement with a chosen analytical cutoff, not proof of whether someone was impaired.

Laboratory confirmation was the reference

Fourteen adults completed placebo, THC-dominant and approximately equal THC/CBD cannabis conditions in a blinded crossover study. Oral fluid was collected before dosing and at scheduled post-use points through three hours.

The investigators compared DrugWipe 5S and DrugTest 5000 results with LC–MS/MS measurements. At a 10 ng/mL confirmatory cutoff, 5% of evaluated DrugWipe results were false positives and 16% false negatives; the corresponding DrugTest proportions were 10% and 9%. These percentages concern test results, including repeated observations from the same people.

Equal THC and CBD did not remove testing variability

Oral-fluid THC generally peaked at the first collection, ten minutes after vaporization, and declined rapidly. Profiles did not differ significantly between the THC-dominant and THC/CBD-equivalent conditions, but individual concentrations varied widely.

Changing the confirmation cutoff changes which measurements count as reference positives and negatives. Invalid tests, the sampling schedule and the selected participant population also affect apparent performance. Results from this experiment cannot simply be transferred to another device version or workplace protocol.

Separate chemical disagreement from functional error

A device-negative specimen above a laboratory cutoff is an analytical false negative under that definition. It is not automatically an impaired driver whom the device failed to identify. Likewise, a reference-negative screening result does not by itself establish a false allegation of impairment.

Forensic interpretation should specify the device, specimen, cutoff and confirmatory procedure. The study evaluated detection of THC in oral fluid; it did not validate either screen as an individual driving-performance test.

Related: Why cannabinoid concentration and impairment require separate interpretation.

Primary research

Detection of Δ9 THC in oral fluid following vaporized cannabis with varied cannabidiol (CBD) content: An evaluation of two point‐of‐collection testing devices. Drug Testing and Analysis (2019).