THC testing of hemp and cannabis is done in a laboratory by chromatography: gas chromatography (GC) or high-performance liquid chromatography (HPLC), sometimes combined with mass spectrometry. The result is given as a percentage of the sample. Many rules now look at total THC, which adds THCA, the acid form of THC in the plant, to the THC already present.
Key facts
- Main methods
- Gas chromatography (GC-FID or GC-MS) and liquid chromatography (HPLC). Rapid immunoassays screen people, not plants.
- EU official method
- GC-FID, the gas chromatography method set by the European Commission for hemp varieties
- Total THC
- THC + (0.877 × THCA); the factor corrects for the weight of the acid group lost on heating
- UK hemp (as of October 2026)
- Licensed growers must use approved seed with no more than 0.2% THC
- UK CBD products (as of October 2026)
- The Home Office expects producers to show compliant levels of Δ9-THC, Δ8-THC and CBN
- Lab-to-lab differences
- Real and documented: a Washington State dataset showed systematic differences between testing laboratories
How is THC measured in a laboratory?
Every method follows the same basic steps:
- Sampling. A representative part of the crop or batch is taken. For plants, flowering tops matter most, because they hold far more cannabinoids than stems.
- Preparation. Plant material is dried and ground; an oil can usually be diluted and analysed directly.
- Extraction. The cannabinoids are dissolved into a solvent.
- Separation and detection. A chromatograph separates the compounds, and a detector measures how much THC comes out, compared against a reference standard.
- Reporting. The result is given as a percentage (grams per 100 g) or in mg per gram or per millilitre.
GC or HPLC: which THC testing method is better?
Both are accepted. The main difference is heat. A gas chromatograph vaporises the sample at high temperature, and at that temperature THCA loses its acid group and turns into THC. So a plain GC reading shows something close to total THC. HPLC works without heating, so it measures THC and THCA separately.
Heat can also distort results in CBD-rich samples. In a 2022 study, a brief pass through the hot GC inlet degraded CBD by 20%, forming Δ9-THC and CBN as by-products, and 17.2% of the Δ9-THC itself degraded. The authors, who note that the European Commission's official method for hemp uses GC-FID, recommend derivatisation (chemically protecting the cannabinoids before analysis) to prevent this (García-Valverde et al., 2022).
| Method | Used for | Strengths | Limits |
|---|---|---|---|
| GC-FID (gas chromatography, flame ionisation detector) | EU official method for hemp varieties; many product certificates | Standardised, widely used | Heat converts THCA (and some CBD) unless samples are derivatised |
| GC-MS (gas chromatography, mass spectrometry) | Forensic work; confirming drug tests | Identifies compounds by their mass spectrum | Same heat issue as GC-FID |
| HPLC (high-performance liquid chromatography) | Cannabinoid profiles of products and flowers | No heating; measures THC, THCA, CBD and CBDA separately | Needs good separation of similar cannabinoids |
| Immunoassay (rapid saliva or urine tests) | Screening people for cannabis use | Fast and cheap | Screening only; positives need laboratory confirmation |
What is total THC?
A growing cannabis plant makes very little THC. It makes THCA, which turns into THC when heated or stored for a long time (what is THCA?). Total THC adds both together. Because THCA is a heavier molecule, labs use a conversion factor. US hemp rules, for example, define it as:
Total THC = (0.877 × THCA) + THC
The same rules describe total THC as "the potential total delta-9 tetrahydrocannabinol content derived from the sum of the THC and THCA content" (7 CFR 990.1). When you compare certificates, check which figure is reported: Δ9-THC only, or total THC.
How are hemp and CBD products tested for THC in the UK?
- Hemp crops: UK growers need a Home Office licence and must use approved seed varieties with a THC content of no more than 0.2%. The licence does not cover the leaves and flowers (Home Office, 2025).
- CBD products: the Home Office says that "Pure CBD, as an isolated substance, is not controlled", but that it is "nearly impossible" to make CBD without traces of controlled cannabinoids such as THC. For a product to fall under the exempt-product rules, producers "should be able to demonstrate compliant levels of Δ9-THC, Δ8-THC, and CBN", and no one component part may contain "more than one milligram of the controlled drug" (Home Office, 2024).
For how these limits separate hemp from marijuana, see cannabis vs marijuana vs hemp, and for the product rules, is CBD legal in the UK?
Why can two labs give different THC results?
An analysis of Washington State's seed-to-sale data "documented systematic differences in the cannabinoid content reported by different laboratories", which persisted after controlling for other explanations. The authors called for standardised lab methods (Jikomes and Zoorob, 2018). The usual reasons are:
- Sampling: flower tops hold far more THC than leaves or stems.
- Method: GC heat converts THCA and some CBD; HPLC does not.
- Calculation: some reports show Δ9-THC only, others total THC.
- Uncertainty: every result has a margin of error.
- Storage: THC breaks down over time, and CBN is one of its breakdown products.
Is a THC test on a person the same as testing a product?
No. Drug tests look for THC or its breakdown products in a person's urine, saliva or blood. In the UK, police "can also use a roadside drug kit to screen for cannabis and cocaine" (GOV.UK). In a study of chronic daily cannabis smokers who then stopped, THC in saliva was generally measurable for 48 hours (Lee et al., 2011). For how this affects CBD users, read CBD, drug tests and driving in the UK.
How can I check the THC content of a CBD product?
Ask for the certificate of analysis (COA) for the batch you bought. A useful COA names the lab, the sample and the batch, the method used, and Δ9-THC and THCA (or total THC), with units and the limit of quantification. Our guide to reading a CBD certificate of analysis explains each line, and the reports for our products are on our test results page.
From our production
We test every batch in Switzerland, and we have never had a failed test since 2013. We print the THC content to two decimal places, and I sign every certificate of analysis myself.
— Robin Roy Krigslund-Hansen, co-founder and CEO
Frequently asked questions
How are cannabis and hemp tested for THC?
A sample is dried or diluted, the cannabinoids are extracted with a solvent, and the extract is separated by gas or liquid chromatography. A detector measures the THC against a reference standard, and the result is given as a percentage.
What is the most accurate THC testing method?
Validated chromatography methods, GC or HPLC, often with mass spectrometry. HPLC measures THC and THCA separately without heating; GC needs derivatisation to do the same. Results also depend on sampling and the lab.
What is the difference between THC and total THC?
THC is the amount of Δ9-THC already present. Total THC adds the THC that THCA would form when heated, calculated as THC + 0.877 × THCA.
Can a home test kit measure THC in hemp?
Colour tests and rapid kits can show that cannabinoids are present, but they are not precise enough to check a 0.2% or 0.3% limit. That needs a laboratory using a validated chromatography method.
Why do THC results differ between laboratories?
Because of sampling, the method (GC heat converts THCA and some CBD), whether Δ9-THC or total THC is reported, measurement uncertainty and storage. A Washington State study documented systematic differences between labs.
Related guides
This article is general information about laboratory testing and the law. It is not legal advice, and rules change. Formula Swiss CBD oils are sold as cosmetics for use on the skin. Read our full disclaimer.
Last reviewed:
Sources
- García-Valverde MT, et al. Effect of temperature in the degradation of cannabinoids: from a brief residence in the gas chromatography inlet port to a longer period in thermal treatments. Frontiers in Chemistry 2022. doi:10.3389/fchem.2022.1038729
- 7 CFR § 990.1, Definitions ("Total THC", "Decarboxylated"). Legal Information Institute, Cornell Law School.
- Home Office. Low THC cannabis (industrial hemp) licensing factsheet. GOV.UK, updated 18 December 2025.
- Home Office. Drug licensing factsheet: cannabis, CBD and other cannabinoids. GOV.UK, updated 3 October 2024.
- Jikomes N, Zoorob M. The cannabinoid content of legal cannabis in Washington State varies systematically across testing facilities and popular consumer products. Scientific Reports 2018. doi:10.1038/s41598-018-22755-2
- GOV.UK. Drugs and driving: the law.
- Lee D, et al. Oral fluid cannabinoids in chronic, daily cannabis smokers during sustained, monitored abstinence. Clinical Chemistry 2011. doi:10.1373/clinchem.2011.164822
