This is the gold standard for scientific studies and it shows that THC concentrations in urine or saliva cannot determine plasma THC levels which could indicate the level of inebriation from cannabis.
Purpose. A randomized cross-over, double blind placebo controlled study of smoked cannabis was carried out on occasional cannabis smokers.
Conclusion. Prediction of plasma THC concentration from THC oral fluid concentration or from THC-COOH urinary concentrations is not feasible due to the large variations observed.