Cannabis Blood Assay Validation Across Laboratories Still Required Authentic-Sample Checks
A commercial blood-testing kit showed broadly similar performance across participating laboratories, but the study also identified a critical limitation of blank-matrix validation: an interferent appeared in authentic cannabis-exposed specimens.
What multicenter validation tested
The investigators compared LC–MS/MS performance for THC and metabolites across laboratory sites using different instrument platforms. Their assessment included calibration, precision, accuracy, interference, carryover, matrix effects and measurement uncertainty.
This was analytical validation rather than a human impairment experiment. Similar laboratory performance supports reproducibility of the measurement process; it does not establish the meaning of a concentration for an individual driver.
An interfering compound mattered in real specimens
The method separated an interferent with mass-spectrometric characteristics overlapping THCCOOH. The paper reported that the interferent occurred in specimens from cannabis users and would not be represented by testing drug-free blank blood alone.
If it co-eluted with THCCOOH, confirmation ion ratios could fail. Thus, a clean blank and a highly linear calibration curve were not sufficient substitutes for chromatographic separation in authentic material.
Performance was not identical on every instrument
Most laboratories supported a broad calibration range, while some instrument configurations required a lower upper limit. A laboratory adopting the kit still needed evidence appropriate to its own instrument, range and casework.
The authors disclosed commercial relationships with products used in the study. Independent replication and local validation remain relevant without automatically invalidating the reported results. A case review should examine actual chromatograms, identification rules and quality controls rather than treating a kit name or accreditation reference as proof that every possible interference was excluded.
Related: Why treatment outcomes require attention to study design.
Primary research
Multi-laboratory validation of a Δ9-tetrahydrocannabinol LC-MS/MS test kit designed for quantifying THC and marijuana metabolites in blood. Forensic science and criminology (2018).
