THC-COOH concentrations in oral fluid and plasma were significantly correlated after repeated oral THC. The authors nevertheless concluded that oral-fluid cannabinoid concentrations could not predict concurrent plasma concentrations.
The correlation involved transformed data
Ten male daily cannabis smokers received increasing-frequency dronabinol capsules over eight days. The correlation between logarithms of oral-fluid and plasma THC-COOH was 0.63, with a p value below 0.001. Parent THC showed a different pattern: its log concentrations were not significantly correlated across the matrices.
The statistically significant metabolite association coexisted with substantial between-person variability and much lower metabolite concentrations in oral fluid than plasma.
Association and prediction have different requirements
A correlation summarizes how observations vary together. It does not specify the error around an individual prediction or establish a universally applicable partition factor. Log transformation also means the reported coefficient should not be mistaken for a slope that converts one raw concentration into another.
The forensic implication
A saliva THC-COOH result can inform exposure interpretation without providing a substitute blood result. It cannot inherit a parent-THC conversion or an impairment threshold from the significant metabolite correlation. This small, high-frequency oral-dosing experiment concerned established users, so its association also requires caution when discussing occasional users, other formulations or different sampling times.
Related reading: cannabis screening and impairment.
Research cited
Milman and colleagues (2011). Oral Fluid and Plasma Cannabinoid Ratios after Around-the-Clock Controlled Oral Δ9-Tetrahydrocannabinol Administration. DOI: 10.1373/clinchem.2011.169490.
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