CBD-THC-Ratio (Metaboliten) in Serum

Serum, 1ml

With the increasing commercial availability and rising consumption of CBD-containing products, healthcare professionals are increasingly faced with the challenge of assessing whether a positive cannabis drug screening test is attributable to the consumption of a CBD product or to the use of marijuana, medical THC, or other THC-containing products. This issue is also relevant for Switzerland, where legally marketed CBD products may contain less than one percent by weight of the active substance Δ9-THC. Thus, for example, a Δ9-THC content of 9.9 mg per gram of CBD cannabis flowers is permitted (see, inter alia, the website of the Federal Office of Public Health [FOPH], legislation on hemp and cannabis products, low-THC hemp, published online on 4 March 2025).

Irrespective of this, such products are also available via the Internet, without any guarantee that they correspond to the declared labeling. In such cases, higher Δ9-THC contents may indeed be detected (see also Habel et al., Positive Cannabis-Urintests durch kommerzielle Cannabidiol-Produkte, Toxichem Krimtech 2020; 87(1):10).

After determination of THC and CBD, including their metabolites (excluding glucuronides, so-called “free” cannabinoids in serum), sums are calculated and a quotient is derived from these values. This quotient is used to estimate the type of hemp products consumed (CBD-rich versus THC-rich products). The collected data are classified into consumption probabilities using a naïve Bayes classifier (e.g., CBD use, combined CBD/THC use, THC use, or insufficient interpretative value). The classifier is based on the approaches described in publications [1–5].

Interpretation is permissible only in conjunction with clinical data. In addition, absolute concentrations (e.g., very high THC-COOH values) must be taken into account, as the quotient may have limited interpretative value in individual cases, particularly in the presence of high serum THC-COOH concentrations (e.g., > 70 µg/L; so-called “heavy smokers rule-in cut-off” according to Fabritius et al., DTA 2014, doi.org/10.1002/dta.1581).

[1] Abschlussbericht CBDDrive: CBD Cannabis und Fahrfähigkeit, Laura Egloff, Priska Frei und Eva Scheurer, IMR Basel, Schweiz und Bundesamt für Gesunhdeit (BAG), Direktionsbereich Verbraucherschutz, 30.11.2021
[2] S. Pichini et al., Δ9-Tetrahydrocannabinol and Cannabidiol Time Courses in the Sera of "Light Cannabis" Smokers: Discriminating Light Cannabis Use from Illegal and Medical Cannabis Use, Ther Drug Monit 2020; 42(1): 151-156
[3] R. Pacifici et al., THC and CBD concentrations in blood, oral fluid and urine following a single and repeated administration of “light cannabis”, Clin Chem Lab Med 2020, 58(5): 682-689
[4] T.J. Gelmi et al., Impact of smoking cannabidiol (CBD)-rich marijuana on driving ability, Forensic Sciences Research 2021, Vol. 6, No. 3, 195-207
[5] A. F. Lo Faro et al., UHPLC–MS-MS Determination of THC, CBD and Their Metabolites in Whole Blood of Light Cannabis Smokers, Journal of Analytical Toxicology, Volume 46, Issue 9, November 2022, Pages e256–e261