In vitro stability of free and glucuronidated cannabinoids in blood and plasma collected in plastic gray-top sodium fluoride tubes following controlled smoked cannabis

Blood and plasma cannabinoid stability is important for test interpretation and is best studied in authentic rather than fortified samples. Previous studies employed green-top (sodium heparin) tubes; however, gray-top (sodium fluoride-potassium oxalate) tubes are generally used in forensic cases. Three participants each provided three low and high blood and plasma pools after smoking a 6.8 % Δ⁹-tetrahydrocannabinol (THC) cigarette ad libitum over 10 min. THC, 11-hydroxy-THC (11-OH-THC), 11-nor-9-carboxy-THC (THCCOOH), and THCCOOH-glucuronide stabilities in samples collected via gray-top tubes were determined after 1 week at room temperature (RT), 1, 4, 12, and 26 ± 2 weeks at 4 °C and 1, 4, 12, 26 ± 2, and 52 ± 4 weeks at −20 °C. Concentration changes <±20 % were considered stable. In blood, THC, 11-OH-THC, and THCCOOH were stable for 1 week at RT, while THCCOOH-glucuronide decreased >47 %. In blood at 4 °C, THC, 11-OH-THC, THCCOOH-glucuronide, and THCCOOH were stable for 26, 26, 1, and 26 weeks, respectively, and for 26, 52, 12, and 52 weeks respectively, at −20 °C. In plasma, THC and 11-OH-THC were stable for 1 week at room temperature, while THCCOOH-glucuronide decreased and THCCOOH increased. In plasma at 4 °C, THC, 11-OH-THC, THCCOOH-glucuronide, and THCCOOH were stable for 12, 26, 1, and 4 weeks, respectively, and at −20 °C for 52, 52, 12, and 52 weeks, respectively. Blood and plasma specimens collected via gray-top tubes should be stored at −20 °C for ≤12 weeks to assure accurate quantitative results of all major and minor cannabinoid markers.