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-Δ9-tetrahydrocannabinol (THC), low extraction efficiencies have been reported in previous studies. In this study, a novel extraction method was developed comprising of an extraction with dimethyl sulfoxide followed by methanolic extraction, yielding extraction efficiencies > 80% for both THC and its metabolite 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC-COOH). The linear ranges of the LC-MS/MS method are 0.5 ng/mL – 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).
Bantle et al. (Tue,) studied this question.