Adsorption of Tetrahydrocannabinol (THC), Metabolites, and Related Cannabinoids During Storage of Plasma Samples in Gel Separation Tubes

The concentrations of some drugs in biofluids can be affected by different storage conditions, including the type of sample collection tube. This phenomenon has been observed in gel separation tubes, where drug adsorption to the gel separator can lead to the underestimation of the drug`s concentration, thus potentially affecting the interpretation of analytical results. The purpose of this study was to determine if concentrations of tetrahydrocannabinol (THC), its metabolites, and related cannabinoids decrease over time when stored in plasma separation tubes (PSTs) as compared to non-PSTs. Plasma samples with a high concentration [HP-24 ng/mL THC, 150 ng/mL carboxy-THC (THC-COOH), 48 ng/mL hydroxy-THC (THC-OH), 15 ng/mL cannabidiol (CBD), and 15 ng/mL cannabinol (CBN)], and low concentration [LP-5.0 ng/mL THC, 31 ng/mL THC-COOH, 10 ng/mL THC-OH, 3.1 ng/mL CBD, and 3.1 ng/mL CBN] of cannabinoids were stored in PSTs and non-PSTs for analysis by liquid chromatography-tandem mass spectrometry at one-hour, three-day, one-week, two-week, three-week, one-month, two-month, and three-month intervals. Statistically significant differences in cannabinoid concentrations (p < 0.05) were observed between non-PSTs and PSTs. All cannabinoids except THC-COOH showed a greater reduction in concentration when stored in PSTs compared to non-PSTs. In contrast, THC-COOH showed an increase in concentration when stored in PSTs compared to non-PSTs. Over a three-month period, concentrations in PSTs decreased for THC by 84% and 81%, THC-OH by 66% and 63%, CBD by 69% and 62%, and CBN by 75% and 70%, in LP and HP samples, respectively. In conclusion, for forensic cases involving cannabinoids, the adsorption of these compounds should be considered in the toxicological interpretation of samples collected in PSTs.