Close THC-COOH Agreement Did Not Validate a Parent-THC Conversion Between Blood Matrices
THC-COOH agreed more closely between dried capillary spots and venous blood than parent THC did. Good agreement for the metabolite did not establish interchangeability for the psychoactive parent compound.
The analytes had different ratio distributions
The study reported a median dried-spot-to-venous ratio of 1.09 for THC-COOH, with a range of 0.60–1.64. Parent THC had a much wider range, including extreme observations. The comparison therefore depended on which chemical was being measured, despite specimens coming from the same general collection exercise.
A metabolite can answer another exposure question
THC-COOH is an inactive metabolite. Its agreement across these specimen types concerns analytical comparability, not whether it reproduces parent THC concentrations or current drug effects. A narrow metabolite ratio cannot repair an uncertain parent measurement by assumption.
Validate the requested analyte directly
For a proposed matrix substitution, ask whether the relevant compound was evaluated under comparable collection and processing conditions. Do not extend a successful metabolite comparison to every cannabinoid in a panel. The results also came from recreational users sampled without a standardized interval after last use. They were not a controlled impairment study or a validated way to reconstruct a person’s functional state from a metabolite result.
Related analysis: the study’s broader findings and limitations.
