The two analytical workflows introduced deuterated THC at different preparation stages. An internal standard added to an extract cannot directly track everything that happened before that addition.
The method difference was explicit
For the oral-dosing group, the deuterated internal standard was added directly to the patch before extraction. In the other laboratory’s procedure, it was added to an aliquot of patch extract before solid-phase extraction. Extraction volume and the aliquot taken forward also differed.
The paper reported the same quantification limit and comparable assay-performance measures, while describing those procedural distinctions.
Timing determines which steps are monitored
An internal standard can help correct for variation after it enters the workflow. Earlier losses require separate validation or controls if they are to be evaluated. This principle does not demonstrate that the two laboratories produced systematically different results; it specifies what the addition stage can and cannot monitor.
Use the whole validation record
Blind fortified quality-control patches helped assess performance across the tested procedures. A case review should examine those results alongside the actual method, rather than assuming that the mere presence of a labeled standard eliminates every possible preparation loss. Analytical agreement also does not validate a conversion from patch THC to blood concentration or impairment.
Related reading: cannabinoid assay validation.
Research cited
Huestis and colleagues (2007). Excretion of Δ9-Tetrahydrocannabinol in Sweat. DOI: 10.1016/j.forsciint.2007.04.002.
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