A Modest THC–THC-COOH Hair Correlation Did Not Provide a Conversion Formula
THC and THC-COOH hair concentrations were positively correlated in the documented-user study, but the association was modest. It did not make one analyte a reliable numerical substitute for the other.
The correlation had a defined analysis set
The authors reported a Pearson correlation of 0.38, with a statistically significant p value, after excluding an observation above the parent-THC reporting range. The finding concerned concentrations in hair specimens under this analytical protocol, rather than paired blood measurements or contemporaneous performance.
Statistical association does not mean close agreement
A positive correlation indicates that larger values tended to occur together. It does not establish equality, a fixed ratio or small prediction errors for an individual. The study also included specimens with one analyte detected without the other, reinforcing the importance of measuring the requested compound directly.
Do not reconstruct an unmeasured result
A significant p value cannot supply a missing metabolite concentration or validate an ingestion-time estimate from parent THC. Sampling, incorporation, analytical thresholds and external exposure remain relevant interpretive issues. The paper supports describing a relationship in this dataset, with considerable unexplained variation. It does not establish a universal parent-to-metabolite hair conversion or show how impaired a person was during a particular event.
Related analysis: the study’s broader findings and limitations.
Research cited
Huestis and colleagues (2006). Cannabinoid Concentrations in Hair from Documented Cannabis Users.
