A Cannabis Urine Screen’s 98.1% Agreement Coexisted With 50% Sensitivity
High agreement can coexist with missed positive specimens when most samples are negative. A cannabinoid urine-screening result therefore needs its cutoff and underlying counts before an “accuracy” percentage can be interpreted.
What the pilot’s comparison actually counted
Spindle and colleagues studied six adults in a crossover experiment. After vaporized cannabis containing 100 mg CBD and 3.7 mg THC, Table V compared urine immunoassay results with LC–MS/MS THC-COOH confirmation at 15 ng/mL.
At the 50 ng/mL screening cutoff, the counts were two true positives, 104 true negatives, two false negatives and no false positives: 98.1% agreement, but 50% sensitivity. The four confirmation-positive specimens came from two people.
Agreement and sensitivity answer different questions
Agreement counts correct classifications across all specimens. Sensitivity considers only specimens positive by the reference procedure. A large negative majority can dominate the former without improving the latter. These measures are not contradictory, and neither should be substituted for the other.
Repeated specimens also require care: sample-level performance is not automatically an estimate based on the same number of independent people. A useful report should identify both denominators and state whether its intended application concerns specimen detection, person-level exposure or something else.
The cutoff changes the comparison
Keeping confirmation fixed, the study’s 20 ng/mL screen detected all four positive specimens but added five false positives. The 100 ng/mL screen missed all four. These were descriptive results, not impairment measurements.
A review should request the full classification table, assay identity, decision thresholds and sampling schedule. It should also distinguish a negative screen from a negative result under a different confirmatory procedure. One percentage cannot communicate that entire testing pathway.
Keep the study’s scope attached
This small, single-dose human urine study did not establish performance for every product or chronic use. Pure-CBD conditions produced no confirmation-positive specimens at the studied threshold. The pilot also overlaps the later expanded report discussed in our article on urinary CBD profiles; these are not wholly independent replications.
Primary research
Spindle and colleagues, Journal of Analytical Toxicology (2020), 44:109–125. DOI: 10.1093/jat/bkz080.
