CBD-THC-Ratio (Metaboliten) in Serum

Serum, 1ml

With the increase in commercial availability and use of CBD-containing products, healthcare providers are increasingly faced with the challenge of determining whether a positive drug test for cannabis was caused by the consumption of a CBD product or by the consumption of marijuana, medical THC or other THC products. This is particularly relevant for Switzerland, as legal CBD products here may contain up to less than one per cent by weight of the active ingredient d9-THC, i.e. an active ingredient content of 9.9 mg d9-THC in one gram of CBD cannabis flowers may be legally sold (see, for example, the website of the Federal Office of Public Health in Switzerland, BAG, Legislation on Hemp and Cannabis Products, Low-THC Hemp, published online on 4 March 2025). Regardless of this, such products can also be ordered online. This means that there is no guarantee that the product actually corresponds to the declaration. Higher levels of d9-THC could well be found in such products (see also S. Habel and colleagues, Positive cannabis urine tests due to commercial cannabidiol products, Toxichem Krimtech 2020; 87(1):10).
After measuring THC and CBD, including their metabolites (excluding glucuronides, known as ‘free’ cannabinoids in serum), the totals are calculated and a quotient is formed. This quotient is intended to provide an estimate of the cannabis products consumed (CBD-rich or THC-rich products)*.
The data is checked for consumption probabilities using a naive Bayes classifier (e.g. CBD consumer, CBD/THC hemp mixed consumption, THC consumer, or no significance). The classifier is based on the following publications [1-5].
*Interpretation only possible in conjunction with clinical data. Absolute concentrations (e.g. very high THC-COOH values) must also be taken into account, so that the ratio is not sufficiently meaningful in individual cases if, for example, THC-COOH values in serum are high (e.g. greater than 70 ug/L; so-called ‘heavy smokers rule-in cut-off’, according to Fabritius et al., DTA 2014).

[1] Abschlussbericht CBDDrive: CBD Cannabis und Fahrfähigkeit, Laura Egloff, Priska Frei und Eva Scheurer, IMR Basel, Schweiz und Bundesamt für Gesunhdeit (BAG), Direktionsbereich Verbraucherschutz
[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