Determination of THC and THC‐COOH in Dried Capillary Blood Spots and Comparison to Venous Blood of Recreational Cannabis Consumers in a Pilot Study

With increased sensitivity of instrumentation, more substances can be analysed using capillary dried blood spots (DBS), and therefore, minimal invasive sample collection can be performed. However, for cannabinoids such as (−)‐ trans ‐Δ 9 ‐tetrahydrocannabinol (THC), low extraction efficiencies have been reported in previous studies. In this study, a novel extraction method was developed comprising an extraction with dimethyl sulfoxide followed by methanolic extraction, yielding extraction efficiencies > 80% for both THC and its metabolite 11‐nor‐9‐carboxy‐Δ 9 ‐tetrahydrocannabinol (THC‐COOH). The linear ranges of the LC–MS/MS method are 0.5–20 ng/mL for THC and 1.25–100 ng/mL for THC‐COOH with good precision and accuracy. Concentrations of THC and THC‐COOH in DBS from capillary blood were compared to venous blood concentrations based on 159 blood samples retrieved from a study including recreational cannabis users. THC‐COOH concentrations in DBS were in close agreement with venous blood concentrations with a median capillary DBS/venous blood ratio of 1.09 (mean 1.10 ± 0.21), whilst THC concentrations in capillary blood were higher than venous blood concentrations with a median ratio of 2.11 (mean 16.6 ± 87.9) (after exclusion of outliers: 1.84 [mean 2.57 ± 1.93]), and the THC concentration ratio capillary DBS/venous blood showed a larger variability, ranging from 0.75 to 722 (0.75 to 8.18 after exclusion of outliers).