Canceled THC Confirmations Were an Isomer-Interference Problem, Not Negative Drug Tests
When a laboratory cancels THC confirmation because an interfering isomer prevents reliable interpretation, the outcome is unresolved, not a negative drug test. A casework study of delta-8 THC illustrates why the reason for a canceled result matters.
An analytical problem emerged in casework
Chan-Hosokawa and colleagues described increasing interference involving delta-8 and delta-9 THC and related metabolites in driving-investigation specimens. Testing cancellations reached a reported monthly peak of 3.7% in February 2019.
The issue was overlapping analytical signals. It prevented the laboratory from confidently applying its reporting procedure; it did not establish that the specimen contained no cannabinoid. Nor did cancellation alone establish which isomer was present.
Method development addressed separation
The investigators developed and validated an updated two-dimensional LC–MS-MS approach with a longer run to separate the compounds. They subsequently reported an interference incidence below 0.5%.
The earlier cancellation rate and later interference figure describe the laboratory’s experience under different procedures. They should not be transformed into estimates of delta-8 use in the general population or a claim that the revised method eliminates every possible interference.
Read the disposition and its reason together
A negative result, a canceled analysis and an unquantifiable identification carry different meanings. Before interpreting an old report, determine what was tested, which reporting criteria failed and whether a result was ever issued.
An updated assay does not retroactively generate a concentration for a previously unresolved specimen. Any later testing would need its own documented result. Even successful isomer identification remains exposure evidence; it is not a direct measurement of driving impairment.
Related: How the reference specimen changes a cannabis accuracy claim.
Research cited
Chan-Hosokawa A; Nguyen L; Lattanzio N; Adams WR; Journal of Analytical Toxicology; 2022. Emergence of Delta-8 Tetrahydrocannabinol in DUID Investigation Casework: Method Development, Validation and Application.
