Positive for cannabis – were they smoking it or just in the room?

Cannabis remains the most commonly used illicit drug in the world1 and usage reports show that cannabis is by far the most prevalent drug in the UK2 and Europe3. It is somewhat ironic that the most prevalent drug is also one of the most difficult to detect in hair. Following cannabis use, the main active ingredient, Δ9-tetrahydrocannabinol (THC) and particularly its main breakdown product are present at very low concentrations within hair.

What happens to cannabis in the body?

The main active ingredient of cannabis is Δ9-tetrahydrocannabinol (THC). It is processed in the body to form its carboxylic acid metabolite THCCOOH.

Metabolism of THC

As hair is on the outside of the body, a hair sample is susceptible to exposure from anything that is in the environment. An individual may well claim that a positive cannabis hair test has resulted from them being in an environment heavy with cannabis smoke. It is therefore important that any laboratory testing hair samples for cannabis can differentiate between passive exposure and active use.

To ensure that this differentiation can be made, the Society of Hair Testing (SoHT) has issued guidelines for testing laboratories to follow. These guidelines specifically include four things to help distinguish between cannabis use and external contamination:

  • washing the hair sample before testing to remove external contamination
  • the analytical sensitivity required to find parent THC
  • the main cannabis metabolite THCCOOH must be present
  • the analytical sensitivity required to reliably detect THCCOOH

Our laboratory process includes a wash step before any testing is conducted. This removes cannabis and other contaminants that are on the outside of the hair, ensuring we only detect drugs that are bound in the hair itself.

The SoHT guidelines specify that laboratories must be able to detect the presence of THCCOOH at levels as low as 0.2 pg/mg of hair. This means detecting about 2 billionths of a gram in an average hair sample! Due to our continued investment in state-of-the-art equipment, we are one of the few laboratories in the UK with the capability of detecting the main cannabis breakdown product at these extremely low concentrations.

Using our laboratory to carry out hair testing for cannabis gives you a fourfold assurance:

  • the hair samples are washed to remove external contamination
  • the THC analysis is sensitive enough to find the parent THC
  • the analysis detects the main cannabis metabolite THCCOOH
  • THCCOOH can be detected at the low levels recommended by the SoHT.

This means that individuals who have been passively exposed to cannabis smoke will not be unfairly accused of cannabis use and regular cannabis users will not go undetected.

Additionally, our analysis is covered by our accreditation to the international laboratory testing standard ISO17025, which gives you the assurance of our competence and capability to carry out this testing.


1 United Nations Office on Drugs and Crime, World Drug Report 2014

2 Drug misuse: Findings from the 2013/14 Crime Survey for England and Wales

3 EMCDDA, European Drug Report 2015

This entry was posted in Drug Testing, Family Law, Substance Misuse. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *